From a technical standpoint, Apple can do whaetver it wants with the iPhone. Hell, they could make an iPhone work on Nextel's iDEN if they wanted to.
But that's the thing... do they want to?
Verizon management has said the ball is in Apple's court. This is code speak for: Apple has made conditions and Verizon isn't willing to accede to them. Both companies have inflated egoes; we've known this for a long time now.
There is also the form-over-function thing: if it costs Apple too much, or it's too impractical to cram a CDMA chipset in with the rest of the radios in their design, then they won't do it unless and until something makes it worth their while. And I doubt they're about to make a CDMA-only version and have separate production lines. The only iPhones that are coming out differently from all the others are China Unicom models, and at best that's to delete WiFi from the package, not to add something different.