We recently tried using php 5.1 on our installation, granted we are still running 1.4.7 (yes, yes, yes, we are in the process of upgrading) and things mysteriously stopped working.... Like rollbacks and the spam blacklist. It was weird. I dug into it and did find that array1 += array2 no longer worked as expected, which was causing our Spam Blacklist extension to fail. It wasn't a fun experience. Going back to php 4.1 returned things to normal.
Travis