Also of note for developers is Apple's preview of Xcode, a pumped-up reworking of Project Builder with geektastic stuff like distributed compilation (coordinated via Rendezvous), predictive compilation (compiling while you edit), and gcc 3.3 behind it all. [UPDATE: meatier page with more info for developers is here.]

This and other cool stuff unveiled at WWDC is summarized in this Macintouch report among other places.