I have the same problem. I just updated to 3.0 on a 2g iPhone. Even before i jailbroke and unlocked the phone (which I do despite being on AT&T because of extensive foreign travel), i noticed that my emails (which I get via Outlook) do not push. When it starts to download mail, which it recognizes is there, it disconnects from the application and returns to the home screen.
Very frustrating!
Any clues? Any fixes?