I've been largely unsatisfied with my Blackberry Curve and have wanted an iPhone for some time, so come August when my plan with Verizon expires, I'll most likely be making the switch.
I'd much rather stay with Verizon, but if they aren't supporting the phone I don't know that I'm willing to. They have great coverage here in L.A. and back home in D.C., but the main reason I stuck with Verizon at the time was because of how many people I knew who were on it ... my entire family, my last three girlfriends ... now though it just doesn't make any sense to stay. My family is all on T-Mobile, and I don't talk to any of the exes anymore.
Really the only thing that would irritate me would be if I make the switch and then shortly thereafter, Verizon begins supporting iPhone. That would be both ironic and frustrating.