I can't speak for all carriers but I know you can purchase any current Cingular handset to use on any prepaid plan. The store has special prepaid pricing for handsets. Additionally (assuming it is a compatible handset) you can use any unlocked device for prepaid service.
i've been looking at pre-paid plans (hate the hassle of bills) and i'm fairly confident i'll choose a pre-paid plan. the problem is, all the phones offered with all the major providers are way too basic. i was wondering if i could purchase whatever phone i like best and use it as my phone with the pre-paid service. would it be as simple as getting the provider to agree to using the phone that i buy independently? if that's even the case, i'm guessing the provider would not be willing to agree. or is there something i could do such as changing the chip in the phone? thanks for any help.