Unlikely.
T-Mobile, the only major GSM competitor in the US, uses a different frequency than the normal GSM standard.
Only phones designed for US T-Mobile will work full speed on its 3g network, otherwise the standard GSM phone will just go to edge speed on T-Mobile.
Verizon is not GSM.
Thus iPhone is probably not going to be dropped from AT&T.
T-Mobile, the only major GSM competitor in the US, uses a different frequency than the normal GSM standard.
Only phones designed for US T-Mobile will work full speed on its 3g network, otherwise the standard GSM phone will just go to edge speed on T-Mobile.
Verizon is not GSM.
Thus iPhone is probably not going to be dropped from AT&T.