WordPress Theme Issues with Php5

Since I upgraded php to php5 the other day I’ve been having some very interesting issues with WordPress themes.
Basically the only theme that will display correctly is k2. The others all give a blank screen and don’t throw any errors in my logs.
Is there something in k2 that the other themes don’t have or vice-versa that makes it compatible with php5?
I’ll be doing some more investigation, but if anyone has any insights I’d really love to hear them, as this issue is really annoying!
EDIT: I finally found the culprit! A plugin I was using to rewrite the post titles was causing issues, so it’s now disabled