If you want iPad with wireless carrier you have to choose your carrier. Are you stuck with the carrier you choose, or does one have ability to use other carriers with the appropriate SIM card? I read that Verizon iPad's have GSM unlocked, so one can use AT&T (and T-Mo??) Still true with new Air? If you get the AT&T, can you also use T-mobile? What about Verizon and Sprint? What about Sprint? Sprint only?