Odds are that a 3G iPhone won't cost any more than an EDGE iPhone, particularly since 3G coverage is sparse compared to EDGE coverage. The exception to my cost theory would be if Apple added some premium features to the 3G iPhone, but 3G itself does not validate a price hike.
So as long as you're not having to pay extra for a 3G iPhone in an area where you can't get 3G and since 3G would automatically switch down to EDGE when 3G is not available, there's no reason to offer additional networks.
And I'm not sure if anyone mentioned it 😉, but Apple has an exclusive contract with AT&T - and the analysts think those kickbacks to Apple are a pretty sweet deal. I don't see Apple breaking that contract.