:^/ In theory, if you can change the PRL on a particular phone, you can flash it to any provider you want. In practice, activating it would be another matter. Using a PRL from a GSM provider on a CDMA phone would be unproductive, (Cricket iPhone, anyone?) -and many carriers won't allow ESN's from phones that they didn't cell you. That's half of why Cricket is so popular with hackers.
;^) Soooo, I see know reason not to support every carrier that will allow foreign ESN's.