and since I'm a Python guy, the Python SDK notes detail a few specific goodies: http://code.google.com/p/googleappengine/wiki/SdkReleaseNotes
I'm excited about support for Django 1.2 and the improve presence capability in XMPP
And, not strictly related to the SDK, from Guido's Tweet stream, something I've wanted for a while:
Check out the Permissions tab on your #appengine Admin Console. New feature: Role dropdown! Owner, Developer, Viewer.