It's almost like the big US carriers are competing for your business all of a sudden. How quaint. AT&T is hoping to lure some new customers by bringing back the $100 bill credit, but only if you sign up for AT&T next.
So here's the deal—this is a limited time offer that is only available through September 30th. You need to buy a phone and activate new service on AT&T Next (the carrier's phone-hopping trade-in program). The bill credit shows up on your account after 3 months of service, assuming you have been keeping up with your payments.
AT&T isn't handing over $100 on the spot when you sign up, which would be much more appealing. Still, if you're undecided about which carrier to go with, this might help you decide.