Two problems . . .
1) ATT generally doesn't allow you to get mileage *after* you sign up for the service. It's an incentive thing, and I'm not aware of any way to start to get miles once you're on the contract.
2) ATT is a terrible service provider. As a next generation customer, I could count on (honestly) at least one hour of hold time every time I tried to contact customer service. I *still* haven't gotten responses to emails I sent to customer service in October. The Verizon phones have fewer bells and whistles, but it's simply not worth the effort to deal w/ ATTs terrible customer attitude.