since i got my new iPhone 3G a few days ago i became painfully aware of the fact that 3G technically means UMTS, which is an entirely new mobile network infrastructure. this means you get all the pains coming with that: poor coverage, unstable antenna availability, and problems with forwarding calls. all of that brought AT&T a lot of bad press already, but in a way i think this is simply what you get when you switch to an entirely new phone network.
on the other hand, the side effects are substantial. for the past three days, i could not receive any calls, callers were always connected to my voicemail. i could call mobile phones, but all calls to landlines failed. obviously, AT&T has massive problems forwarding calls between networks.
and because the UMTS network coverage is so spotty (even here in the east bay area), the theoretical advantage of higher 3G data speeds is often purely theoretical, because you get only an EDGE connection, or even worse, the phone tries to figure out what's going on and constantly switches between 3G and EDGE and then nothing at all happens.
so, while i am still willing to give AT&T some time to get their act together, what i really would like to have is a way to manually configure networks, so that i can set my iPhone to use only the GSM network. because being on 3G so far does not turn out to be a very satisfactory experience...
[[ update September 5: RTFM. Settings → General → Network takes you to a place where you can switch off 3G. ]]