Jobs didn't directly address this at the recent D8 interview, but he did make some relevant comments. Apple really wants to just build one device and ship it anywhere in the world. They don't want to produce phone X for market Y, and phone Z for yet another market. AT&T is the only provider in the US who offers 3G GSM service on frequencies that align with what others use in the rest of the world. T-Mobile 3G uses a frequency setup noone else does, and Sprint/Verizon don't even use a compatible protocol (CDMA).

This will change down the road with AT&T, T-Mobile and Verizon all planning on LTE for 4G at some point, and Sprint hasn't taken the option off the table.

And I doubt Apple is holding to this as the only reason not to go to Verizon or the others. It's just a strong factor. Next week, we may all be talking about the newest iPhone being available on some other carrier. They did after all make one alternate revision for China, iPhones lacking WiFi.