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.

Her solution came in the form of a PHP 5.3 only patch for XDebug and threads. If you're on Windows, you can also grab the patched compile to make things all better again.