When deciding which standard to support, it doesn't really matter if more than half of Americans today are on non-GSM networks. It matters whether 99% of Americans today can use GSM networks.
The vast majority of Americans have access to at least one GSM network, I'd say 90-95% have at least two rival GSM operators in their area. That is, it may be a matter of cancelling your Sprint PCS or Verizon contract and getting a T-Mobile or Cingular one instead, but you can do it. And yes, I'm sure someone will raise the point that it's expensive to "get out" of a contract, but T-Mobile actually buys people out of their contracts anyway, and most people do not replace their phones until near to the end of their contracts.
In other words, yes, Apple can just make this an unlocked GSM phone.
And it's in their best interests to do so, as long as the operators of the "mainstream" alternative insist on requiring the subscriber get permission to connect an arbitrary but compatible phone to their networks. When the IS-95/IS-2000 operators adopt R-UIM, then yeah, I see value in Apple selling a phone for those standards. But for now, switching someone over to an Apple GSM or UMTS phone is going to be infinitely easier, and require far less handholding and involvement of third parties, than switching someone over to a CDMA2000 phone.
Sorry Verizon. This is something you can change, but you're too much of a bunch of control freaks to be willing to think that way.
Sorry Verizon subscribers, you backed the wrong horse. At least be glad competition is strong enough that rivals will buy you out of your contract.
The only bad thing about GSM is that Cingular is the most widespread operator in the US, and they're not doing a great job. With the release of AWS spectrum, I'm expecting that to change, as a significant amount of additional capacity can be added, and competition nationwide from T-Mobile will also make a major difference.