If you think about it logically from a business perspective from Apple's side:
90 million VZN customers
45 million Sprint customers
33 million T-Mobile customers...
RIGHT AT THIS VERY MINUTE
So, you're Apple, and you're looking at 168 million customers who, as of yet, have NOT left their carrier for your product. It's been 3 years now, I think that most of the people who were leaving for the iPhone have. I saw an article TODAY on Engadget from a research firm showing that Android is now the 2nd most used mobile platform behind RIM, unseating the iPhone OS.
We can speak about the "technological difference between GSM and CDMA" all day...and it doesn't change the fact that CDMA handles data traffic MUCH better than GSM. It doesn't change the fact that many other wireless device makers make the same devices for every single carrier. Right now, if you're at any carrier other than ATT you can get Android...and multiple choices of it.
When the exclusivity ends is not the problem...it's what Apple is losing in market share in mobile in the mean time that does matter. ATT has gotten so bad with service that even the late night talk show hosts are slamming them. Perception DOES equal reality.
If the market leader in mobile OS makes devices for two wireless platforms, then Apple can do the same. But choosing to lock yourself up for extended periods of time with one carrier is foolish, and that's starting to show now. Unfortunately for Apple, it will probably still cost them market share even when they do open up to other carriers as folks will already have adapted to Android, which for all intents and purposes, honestly does work better. It seems like Apple is now playing catch-up to Google in the very area where Apple pulled off the reinvention of the product.
Don't get me wrong, I'm not an Apple basher (new MPB, iPad, iPod Touch are all right here on my desk), but from a business perspective IN THE LONG RUN, this exclusivity hurts rather than helps. Android will reach ever single wireless customer in the US, regardless of carrier (I'm speaking about the big 4, not rural carriers) while Apple will continue to only serve the 80 million on ATT right now. That's it. So plain and so simple that any argument that only using 1 wireless technology when two are available is foolish.
And by the way, CDMA is used in many other large countries besides the US. And further, all CDMA carriers have at least a couple of phones that run dual chipsets (CDMA+GSM) to serve their "travelling over seas customers". Apple could pull that off as well and continue to serve US customers travelling abroad.