Well, if it's unlocked, you can just insert whichever prepaid SIM you want. And, if you get a Google Voice number, just have it forward to the number assigned to that SIM, and your number will never have to change.
Of course, this won't work for CDMA carriers, but that's because CDMA doesn't use a SIM, and CDMA carriers have to decide to allow a phone to connect to their network.
