According to this new post on DynamicWebPages.de today, Ilia Alshanetsky has pushed the latest release candidate for PHP 5.1.x series out the door - version 5.1.5RC1.
This release was needed to clean up some of the problems found while working towards PHP 5.2. Among others, there were issues with the URL extension and parts of the Zend Engine 2 that needed correcting.
Complete information and a list of changes can be found in the Changelog and you can download this package here. As always, the development team requests any kind of testing that developers can do on this release candidate to find the bugs quickly.