Very, very few handsets are built for both GSM and CDMA, which is a situation that won't change for quite a while (upcoming next-gen LTE handsets should work on all carriers, but we will see about that).
At the time of the iPhone deal, T-Mobile and AT&T were Apple's only two real options for deployment (Verizon was a negotiating tactic) -- they wanted to build a GSM handset they could deploy globally. The situation would not be any different today. AT&T only won that war because of their dominant position, which allowed them to throw billions at promotions and guarantees -- which, of course, only further worsened T-Mobile's ability to compete (imagine if they had gotten the iPhone instead).
In today's world, if you were Apple, AT&T would once again be able to sit back and dictate terms, as they will be the only real nationwide GSM network in town. It's 1998 all over again, and I don't like it one bit. The other players -- MetroPCS, etc -- are clearly regional / metropolitan plays and do not offer the footprint necessary for an iPhone type device.
As an aside, I would appreciate if your responses were more than one liners.