Don't underestimate the power of the iPhone to attract customers to AT&T's shoddy network.
Have a read of this from today's earnings statement:
3.2 Million iPhone Activations. On June 24, AT&T began offering iPhone 4, the most powerful iPhone yet. Preorder sales of iPhone 4 were 10 times higher than the first day of preordering for iPhone 3GS a year earlier. For the full second quarter, AT&T iPhone activations totaled 3.2 million, the most quarterly iPhone activations ever. Approximately 27 percent of those activations were for customers who were new to AT&T.
The iPhone is now on it's fourth iteration, and it can still manage to pull in a whopping 27% fresh blood to AT&T, despite the bad press that the network has received.
It seems that AT&T can hold onto existing customers pretty well too:
No wonder AT&T loves Apple.