AT&T Wireless now will offer SIM only plans. This opens up the network more easily to unlocked phones.
It also means when you go to their store you no longer will be forced to buy a phone you don't need. It also means that the subsidy of a handset isn't needed, and thus in theory profit per month will be greater for AT&T.
One opportunity they seemed to have not rolled out yet is a special Phone less per minutes pricing plan...of course that would be to logical. They are offering though a discount when the sign-up occurs online.