I am evaluating WordPress and got a hard stop by the requirements: WP requires PHP 4.3 although Support for PHP 4 has been discontinued since 2007-12-31. Please consider upgrading to PHP 5.2.
Looking at the forums, it seems that PHP5 is causing issues with WP which does not surprise me (I have a self-developed PHP application which I had to make PHP5 compliant; it's doable but requires some efforts).
It is a security risk to run an application on an unsupported PHP stack
What are the plans for PHP5?