Yesterday, the PHP team released the latest update to the PHP 5.1.x series, version 5.1.3. Unfortunately, there seems to be some issues surrounding it that you might want to know before making the upgrade.
Filip de Waard writes:
There is a critical bug in the newly released PHP 5.1.3 concerning the $_POST array. There is also a discussion about problems with the combination of PHP 5.1.3, IIS and FastCGI and there are other unconfirmed reports as well. My advice would be to stay away from PHP 5.1.3 until the PHP group releases a fix.
A release should be coming shortly - dmitry has already made an update to the CVS for the release to correct the problem.
UPDATE: the PHP team has released the latest update in the 5.1.x series - PHP 5.1.4 - to correct this issue. Please update ASAP.