The PHP group has officially released the latest update in the PHP 5.1.x series today - PHP 5.1.4, a bugfix release to take care of the $_POST array handling problem from 5.1.3.
A critical bug with $_POST array handling as well as the FastCGI sapi have been discovered in PHP 5.1.3. A new PHP release 5.1.4 is now available to address these issues. All PHP users are encouraged to upgrade to this release as soon as possible.
Due to the wide-spread nature of this bug, updating to this latest version would be a very good idea. There are a few other bugs that were fixed in the release as well including issues with the FastCGI sapi, a fix to a possible crash in hightlight_string, cloning of DOM Documents and Noded, and several fixes to the PDO ODBC driver.
You can download this latest update here.