It looks like Apple will be sticking with AT&T for at least the near future. Why? you ask. Well there are more than a few issues with moving to another carrier but the biggest reason is the negotiated contract rates that Apple gets from AT&T. As it stands right now Apple gets a rather large chunk of the money that people spend on their monthly AT&T plans for the iPhone. This will continue with the iPad as well.
If Apple moved to someone like T-Mobile (who also uses GSM) their much lower rates would not give the same revenue stream. This is something that the accountants at Apple would not like to have happen to their favorite cash cow.
But aside from the monthly money, Apple would have to put some real R&D money into phones to work on other networks. Both Verizon and Sprint (which will have a faster network once they finally get LTE out) are CDMA in the US and GSM in most of the rest of the world with the exception of India and some countries in Africa. This would mean that to Apple would have to make either a global device with both GSM and CDMA capabilities or one for each geographical region. Both of these choices cost money and would impact the margins that Apple so covetously guards on the iPhone.
So in the end I have to agree with many analysts, Apple will not be moving from AT&T in the US any time soon. There is too much (Money) to loose. Of course there is the very slim chance that the FTC will force them to move, but that is a one in a million shot. No, sadly enough iPhone users that are not unlocking their phones in the US are stuck with the shoddy and rather slow AT&T network and overpriced service for some time to come.