As usual, the new beta SDK is under NDA, so please go check out the details for yourself to get further information about the changes between this beta and the last. Good luck, and happy programming!
Update: 6PM Pacific. It's baaaaaaaack.
Update: Here then gone? Readers report that Apple seems to have pulled Beta 3 for the moment and when I returned to Apple's iPhone developer site, I saw the same. Watch this space. We'll update as we find out more.
Update: This was not an accidental upload, per Michael Jurewitz, the Mac and IPhone Applications Frameworks and Developer Tools Evangelist: