Why wouldn't they? I know about a dozen people who have switched to AT&T precisely because they are the only US iPhone game in town (mostly). The exclusivity ends 2010, but SAI asks, will Apple renew its contract with AT&T? The biggest problem plaguing iPhones appears to be the network -- AT&T is notorious for dropped calls and spotty 3G coverage, whereas now-number 1 service provider Verizon has a great reuptation for coverage (both voice and data).
The issue for Apple is compounded by the fact that AT&T uses the globally-accepted network standard of GSM, where Verizon uses the other, less-used standard CDMA. So now you've got two radios to deal with (as provided in the Blackberry 8830 World Edition), something I think Cupertino isn't eager to implement.