As usual, the details of this release remain behind the NDA wall, so you'll need to scoot on over and check them out yourself. Apple will be releasing 4.0 into the general iPhone population this summer. An iPad version will debut in the fall.
The servers typically get slammed whenever a new SDK is released, so you may want to wait a few hours if you encounter any problems connecting to the developer site.
Good luck and happy developing!
Update: Minor bug in the OS 4.0 beta 2 firmware discovered. Workaround: Take screen shots via Xcode.