PHP5 "breaks" theme css

Looking for some advice...

I have a heavily modified theme that has been working fine. I recently made a change from PHP Version 4.4.9 to PHP Version 5.2.6 and the pages render as though the stylesheet is missing.

I realize that diagnosing an entire theme can't be done via this forum, but I'm curious if anyone has some pointers on where to start looking. Note that I have installed only a few non-WPMU Premium plugins, and disabling those does not seem to solve the issue - so I assume the problem lies with a function or some other theme specific element.

Again, just wondering if there are any known issues that I should be looking for.