Let me tell you, I've been waiting for this release. However, I am extremely disappointed that it does not fully support PHP 5.1. I was under the impression that this was the primary purpose of the release.
Oddly enough, the internal debugger DOES support PHP 5.1, it is just the debug server that is limited to PHP 5.0.x and earlier. Of course, since this release is not yet publicly displayed at zend.com (I got it from downloads.zend), perhaps there are plans for the debugger to be released soon independent of the IDE. There is an empty "debugger" directory there that was created on the same day that ZDE 5.1.0 was uploaded, so this is promising. Otherwise, it seems to be an improvement over the old version, noticeably faster even on my fast machine.