Apple made mention that some developers, when updating to iPhone 3.0 Beta 5, might experience problems connecting to AT&T's data network. Apple asked any devs so affected to contact them, and one dev who had the problem called in and let us know what Apple told him:
Apple just called me back, had me turn the phone off then back on. Then turn the wifi off and try it and it seems to be back to working. Thaaaank you apple.
Thanks indeed! If you're experiencing similar problems, you might want to try the above. If that doesn't work for you, however, get on the phone with Apple asap.
(Thanks Josh for the tip!)