Attention, developers! Apple's got the third beta of iPhone OS 4.0 and its SDK available for download here. No word yet on what's changed from the previous release, but if you're registered with the iPhone Dev Center, get to it. If you're one of the rest of us, the official public release is expected this winter. Updated.
Update: Looks as though the camera crashing issues the previous release had have been resolved, for starters. Check out the rest of what's new here.