There have been some changes to internal PHP stuff for 5.3, and some don't always show up unless you're looking at your compile logs and testing with thread safety on.
Currently Xdebug will crash and burn if you're using it with 5.3 and ZTS on - I'm sure Derick will get it fixed when he's back from vacation ;) But not everyone can wait and I needed it sooner so I fixed my local copy. I didn't have the time to do a proper patch yet (use a macro so it compiles against 5.2 as well) but you're welcome to the source. Notice it will only compile against php 5.3.
If you're using windows and testing/playing with 5.3 alpha/beta builds or snapshots and xdebug is crashing for you (most likely because you're trying to use code coverage or pretty var dumps, the two areas where the issues show up) you can grab a patched compile until the next release.