Apple needs Verizon way more than Verizon needs Apple at this point in the game.
-Since the iPhone was released, Verizon has not had a single quarter with negative subscriber growth for postpaid customers.
-Since the iPhone was released, Verizon still has one of the lowest churn rates in the industry.
It's not like Verizon is bleeding customers over the iPhone. If Apple stays with AT&T their overall base won't grow by very much.
Did Verizon miss out on some possible extra growth without the iPhone? Maybe. However business is far more than getting the best numbers possible. Look at AT&T's reputation now. It's pretty crappy. People associate AT&T with subpar service, while Verizon is associates with the most reliable network and the best coverage. While AT&T was busy drowning as their network overloaded Verizon was making upgrades and beefing up their network getting ready for the smartphone revolution. AT&T's growth happened too fast, while Verizon's growth was controlled.
Anyone remember the days of AOL dialup long before broadband? They were the first to offer an unlimited dialup price for a cheap amount. People flocked to AOL, their growth was extraordinary. Great, right? Wrong. It grew too fast and couldn't keep up with demand so while AOL was being killed the other dialup companies like MSN, Compuserve, etc were beefing up their capacity and rolling out unlimited dialup at a more controlled rate. Even after AOL solved their problems they were known as the unreliable dialup provider with a lot of busy signals.
The smartest thing Verizon did was let AT&T get the iPhone. AT&T got to deal with the growing pains and should Verizon get it this summer then they get a tested product and are prepared to handle it.
By the way I'm no Verizon fanboy. In fact, I believe people keep overlooking Sprint. They're the true hidden gem of the cell phone industry in the US in my opinion.