There's a note over on the main PHP site, that the release of the latest version in the PHP 5.1.x series has been posted - PHP 5.1.2.
The PHP development team is proud to announce the release of PHP 5.1.2. This release combines small feature enhancements with a fair number of bug fixes and addresses three security issues. All PHP 5 users are encouraged to upgrade to this release.
Some of the larger changes in this new version include: "HTTP Response Splitting has been addressed in ext/session and in the header() function", "Upgraded OCI8 extension", and "Over 85 various bug fixes".
So, what are you waiting for? Get over and upgrade now!