New on the Zend Developer Zone is an article looking t migrating your application from PHP 5.2 up to 5.3 (relatively) painlessly.

Quick research shows that most prominent applications had to be patched or changed, even though minimally, to work cleanly with 5.3. To help people to convert their web applications to 5.3, I present here a migration script that would allow you to inspect your applications for potential migration problems. Do not expect supernatural wonders from it, but it may save you some valuable time.

The migration script looks for a few things (like bad use of magic methods and the use of deprecated functions) but doesn't catch bad use of reference or, of course, fix the issues for you - you'll still need to make the recommended updates yourself. An example of its use and output is also included.