No one I have spoken to can give me a direct answer to this question, which leaves me with two conclusions: 1) they're internally working out the details on one and will soon release it; 2) they want to see how successful it is for VZW before they respond. I say this because AT&T responds to VZW only; they don't see T-Mobile or even Sprint as competition. It's also possible that they have no plan to respond at all and plans to hold on to rollover like some holy grail of cell phone features.
