You might well be right about that. Forgetting Verizon for a second, in the lack of any sort of good evidence to the contrary, I have a hard time believing the AT&T exclusive will go further than another year. Surely AT&T will have gotten the majority of their potential iPhone-switchers by then. At that point, it's easy to imagine that the benefits for both companies of an exclusive agree would be less clear.
Talk about low-hanging fruit - Tmobile would be an easy market to get into. At that point, I imagine Apple will be asking the following questions of their engineers and market analysts:
1. How much much would it cost us to produce a CDMA iPhone?
2. How many could we sell to Verizon customers and others?
3. Is the profit margin large enough that it's worth it?
These are the questions which nobody here can answer. I imagine that Apple's going to want to get the app store into as many hands as possible, just like with iTunes. That could factor into making CDMA networks more attractive to them. It's not just a phone sale anymore - it's a platform for app sales.
T-mobile uses unusual frequencies for their GSM, and I don't believe the new iPhone cell radio utilizes those frequencies.
I still think the cost of developing a CDMA iPhone is peanuts compared to the money they'd make, especially when you factor in incidental sales of Mac computers. If Apple doesn't have enough engineering resources to do the design work, that's one thing, but if it's purely a matter of cost-benefit, then I think the decision is clear.