I proved that UPDATE time grows with larger number of rows in the table, while SELECT time for the same rows does not change significantly. Here is the entire test case:

[root@Fedora tmp]# /usr/bin/mysqld_safe --log-slow-queries=/tmp/slow.log --log-queries-not-using-indexes &
[1] 2643
[root@Fedora tmp]# Starting mysqld daemon with databases from /var/lib/mysql

[root@Fedora tmp]# /usr/bin/mysql -uroot test
Reading table information for completion of table and column names
You can turn off this feature to get a quicker startup with -A

Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is 1 to server version: 4.1.16-standard-log

Type 'help;' or '\h' for help. Type '\c' to clear the buffer.

mysql> CREATE TABLE `News` (
    ->   `FileID` int(8) unsigned NOT NULL auto_increment,
    ->   `PostID` mediumint(8) unsigned NOT NULL default '0',
    ->   `Subject` varchar(255) NOT NULL default '',
    ->   `Category` tinyint(3) unsigned NOT NULL default '0',
    ->   `SubCategory` smallint(5) unsigned NOT NULL default '0',
    ->   `Date` int(10) unsigned NOT NULL default '0',
    ->   `Parts` smallint(5) unsigned NOT NULL default '0',
    ->   `Bytes` int(10) unsigned NOT NULL default '0',
    ->   `fi_junk` int(10) unsigned NOT NULL default '0',
    ->   `AuthorID` mediumint(8) unsigned NOT NULL default '0',
    ->   `isNFO` tinyint(3) unsigned NOT NULL default '0',
    ->   `SegFirstTime` int(10) unsigned NOT NULL default '0',
    ->   `SegLastTime` int(10) unsigned NOT NULL default '0',
    ->   `Segments` smallint(5) unsigned NOT NULL default '0',
    ->   `SegmentsSeen` smallint(5) unsigned NOT NULL default '0',
    ->   `GroupsHash` mediumint(8) unsigned NOT NULL default '0',
    ->   PRIMARY KEY  (`FileID`),
    ->   KEY `C_D_P_j` (`Category`,`Date`,`PostID`,`fi_junk`),
    ->   KEY `P_j_F` (`PostID`,`fi_junk`,`FileID`),
    ->   KEY `A_j_P_D` (`AuthorID`,`fi_junk`,`PostID`,`Date`),
    ->   KEY `C_P_D_j` (`Category`,`PostID`,`Date`,`fi_junk`),
    ->   KEY `Date` (`Date`),
    ->   KEY `S_D` (`SubCategory`,`Date`),
    ->   KEY `Subject` (`Subject`(10)),
    ->   KEY `C_D` (`Category`,`Date`),
    ->   KEY `C_B` (`Category`,`Bytes`)
    -> ) ENGINE=InnoDB DEFAULT CHARSET=latin1;
Query OK, 0 rows affected (0.16 sec)

mysql> insert into News (PostID) values (1), (2), (3), (4);
Query OK, 4 rows affected (0.00 sec)
Records: 4  Duplicates: 0  Warnings: 0

mysql> insert into News (PostID) select PostID from News;
Query OK, 4 rows affected (0.03 sec)
Records: 4  Duplicates: 0  Warnings: 0

mysql> insert into News (PostID) select PostID from News;
Query OK, 8 rows affected (0.01 sec)
Records: 8  Duplicates: 0  Warnings: 0

... executed the above several times ...

mysql> insert into News (PostID) select PostID from News;
Query OK, 512 rows affected (2.06 sec)
Records: 512  Duplicates: 0  Warnings: 0

mysql> insert into News (PostID) values (1608140);
Query OK, 1 row affected (0.00 sec)

mysql> explain SELECT COUNT(*) FROM News WHERE PostID IN (1608140);
+----+-------------+-------+------+---------------+-------+---------+-------+------+--------------------------+
| id | select_type | table | type | possible_keys | key   | key_len | ref   | rows | Extra                    |
+----+-------------+-------+------+---------------+-------+---------+-------+------+--------------------------+
|  1 | SIMPLE      | News  | ref  | P_j_F         | P_j_F |       3 | const | 1 | Using where; Using index |
+----+-------------+-------+------+---------------+-------+---------+-------+------+--------------------------+
1 row in set (0.04 sec)

mysql> UPDATE News SET PostID = 1608665 WHERE PostID IN (1608140);
Query OK, 1 row affected (0.04 sec)
Rows matched: 1  Changed: 1  Warnings: 0

mysql> exit
Bye
[root@Fedora tmp]# tail /tmp/slow.log
# Time: 060114 13:08:03
# User@Host: root[root] @ localhost []
# Query_time: 1  Lock_time: 0  Rows_sent: 0  Rows_examined: 512
SET last_insert_id=257,insert_id=257;
insert into News (PostID) select PostID from News;
# Time: 060114 13:08:06
# User@Host: root[root] @ localhost []
# Query_time: 2  Lock_time: 0  Rows_sent: 0  Rows_examined: 1024
SET last_insert_id=513,insert_id=513;
insert into News (PostID) select PostID from News;

So, our last update was not logged. Note the time it took. Let's continue: 

[root@Fedora tmp]# /usr/bin/mysql -uroot test
Reading table information for completion of table and column names
You can turn off this feature to get a quicker startup with -A

Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is 2 to server version: 4.1.16-standard-log

Type 'help;' or '\h' for help. Type '\c' to clear the buffer.

mysql> update News set PostID=1;
Query OK, 769 rows affected (1.25 sec)
Rows matched: 1025  Changed: 769  Warnings: 0

mysql> exit
Bye
[root@Fedora tmp]# tail /tmp/slow.log
insert into News (PostID) select PostID from News;
# Time: 060114 13:08:06
# User@Host: root[root] @ localhost []
# Query_time: 2  Lock_time: 0  Rows_sent: 0  Rows_examined: 1024
SET last_insert_id=513,insert_id=513;
insert into News (PostID) select PostID from News;
# Time: 060114 13:11:04
# User@Host: root[root] @ localhost []
# Query_time: 1  Lock_time: 0  Rows_sent: 0  Rows_examined: 0
update News set PostID=1;

For that UPDATE index was surely not used, and it was logged.

[root@Fedora tmp]# /usr/bin/mysql -uroot test
Reading table information for completion of table and column names
You can turn off this feature to get a quicker startup with -A

Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is 3 to server version: 4.1.16-standard-log

Type 'help;' or '\h' for help. Type '\c' to clear the buffer.

mysql> insert into News (PostID) select PostID from News;
Query OK, 1025 rows affected (1.29 sec)
Records: 1025  Duplicates: 0  Warnings: 0

mysql> insert into News (PostID) select PostID from News;
Query OK, 2050 rows affected (2.07 sec)
Records: 2050  Duplicates: 0  Warnings: 0

mysql> insert into News (PostID) select PostID from News;
Query OK, 4100 rows affected (3.06 sec)
Records: 4100  Duplicates: 0  Warnings: 0

mysql> insert into News (PostID) select PostID from News;
Query OK, 8200 rows affected (5.68 sec)
Records: 8200  Duplicates: 0  Warnings: 0

mysql> insert into News (PostID) select PostID from News;
Query OK, 16400 rows affected (11.02 sec)
Records: 16400  Duplicates: 0  Warnings: 0

mysql> insert into News (PostID) values (1608140);
Query OK, 1 row affected (0.00 sec)

mysql> insert into News (PostID) values (1608140);
Query OK, 1 row affected (0.01 sec)

mysql> insert into News (PostID) values (1608140);
Query OK, 1 row affected (0.00 sec)

mysql> explain SELECT COUNT(*) FROM News WHERE PostID IN (1608140);
+----+-------------+-------+------+---------------+-------+---------+-------+---
---+--------------------------+
| id | select_type | table | type | possible_keys | key   | key_len | ref   | ro
ws | Extra                    |
+----+-------------+-------+------+---------------+-------+---------+-------+---
---+--------------------------+
|  1 | SIMPLE      | News  | ref  | P_j_F         | P_j_F |       3 | const |
 3 | Using where; Using index |
+----+-------------+-------+------+---------------+-------+---------+-------+---
---+--------------------------+
1 row in set (0.00 sec)

mysql> SELECT COUNT(*) FROM News WHERE PostID IN (1608140);
+----------+
| COUNT(*) |
+----------+
|        3 |
+----------+
1 row in set (0.01 sec)

mysql> UPDATE News SET PostID = 1608665 WHERE PostID IN (1608140);
Query OK, 3 rows affected (1.43 sec)
Rows matched: 3  Changed: 3  Warnings: 0
mysql> exit
Bye
[root@Fedora tmp]# tail /tmp/slow.log
# Time: 060114 13:11:53
# User@Host: root[root] @ localhost []
# Query_time: 5  Lock_time: 0  Rows_sent: 0  Rows_examined: 16400
SET last_insert_id=8201,insert_id=8201;
insert into News (PostID) select PostID from News;
# Time: 060114 13:12:06
# User@Host: root[root] @ localhost []
# Query_time: 11  Lock_time: 0  Rows_sent: 0  Rows_examined: 32800
SET last_insert_id=16401,insert_id=16401;
insert into News (PostID) select PostID from News;

So, this update of 3 rows out of 16K+ took almost 2 seconds. Surely index is not used, but the UPDATE was not logged. Let's double the number of rows once more:

[root@Fedora tmp]# /usr/bin/mysql -uroot test
Reading table information for completion of table and column names
You can turn off this feature to get a quicker startup with -A

Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is 4 to server version: 4.1.16-standard-log

Type 'help;' or '\h' for help. Type '\c' to clear the buffer.

mysql> update News set PostID=1;
Query OK, 3 rows affected (0.75 sec)
Rows matched: 32803  Changed: 3  Warnings: 0

mysql> insert into News (PostID) select PostID from News;
Query OK, 32803 rows affected (18.20 sec)
Records: 32803  Duplicates: 0  Warnings: 0

mysql> insert into News (PostID) values (1608140);
Query OK, 1 row affected (0.01 sec)

mysql> insert into News (PostID) values (1608140);
Query OK, 1 row affected (0.00 sec)

mysql> insert into News (PostID) values (1608140);
Query OK, 1 row affected (0.01 sec)

mysql> SELECT COUNT(*) FROM News WHERE PostID IN (1608140);
+----------+
| COUNT(*) |
+----------+
|        3 |
+----------+
1 row in set (0.00 sec)

mysql> UPDATE News SET PostID = 1608665 WHERE PostID IN (1608140);
Query OK, 3 rows affected (3.25 sec)
Rows matched: 3  Changed: 3  Warnings: 0

mysql> exit
Bye
[root@Fedora tmp]# tail /tmp/slow.log
insert into News (PostID) select PostID from News;
# Time: 060114 13:14:16
# User@Host: root[root] @ localhost []
# Query_time: 0  Lock_time: 0  Rows_sent: 0  Rows_examined: 0
update News set PostID=1;
# Time: 060114 13:14:41
# User@Host: root[root] @ localhost []
# Query_time: 18  Lock_time: 0  Rows_sent: 0  Rows_examined: 65606
SET last_insert_id=32804,insert_id=32804;
insert into News (PostID) select PostID from News;

So, SELECT is instant, UPDATE took about 2 times more on the data 2 times larger. I believe, it is enough evidence that index is NOT USED FOR UPDATE.