We've had no issues running PHP 5.2.11. 5.2.10 apparently had some issues though but even when running it as a cgi or mod_php that version had crashing problems.
Well I forgot one thing to mention as a warning. Some scripts encoded with very old versions of Zend Guard / Zend Encoder will crash. This typically should not be an issue it seems to be problems with files encoded several years ago and Zend Optimizer causes a PHP crash when it parses them.