You should be able to do that. Although with the iPhone I think they need to add a chip or something because AT&T uses a different kind of network than the other companies. I'm not 100% of that though.
Indeed, there will be two different iPhone models, one for GSM carriers, which would be AT&T and T-Mobile (though T-Mobile may have some issues because of the frequency it's network operates on being different from AT&T/global standard) in the us and 99% of the providers world wide, and one for CDMA networks, which would be Verizon and Sprint as well as many smaller regional carriers and some carriers in Canada, Mexico, and Caribbean.
A third iPhone model is theorized to be rolling out some time in late 2011 which will operate on the WiMax 4g network that is currently live and expanding, though this may simply be a revision of the CDMA based iPhone, as WiMaax is sprint's 4g and sprint already has talked about dual 3g/4g phones. Such a set up could in theory provide a "split service" where data is handled over the 4g wimax network and voice over 3g. This would in theory allow for the same simultaneous voice and data usage that GSM is capable of that AT&T has been marketing the hell out of of late.
But the basic truth it that you
will not be able to just switch service from AT&T to V or S, you will need to get a new CDMA iPhone...
How it seems that the carrier change is set to line up with the introduction of the latest generation of iPhone anyway, so it isn't like you are paying twice for the same thing.
From my internet sleuthing, I've seen July as the date of announcement of the new iPhone and new carriers, with some time in December as the start of sales for the new iPhone across the market. Dec 11 and Dec 21 are the most common mentioned dates for this, which would place it just in time for it to the the hot xmas item this year.
