Had Verizon for like 7 years and switch to AT&T for the iphone on July 11. AT&Ts service sucks really bad compared to verizon everywhere I've used it. And I live in the DC metro area - decent size metro area! AT&T is pretty good in the metro area itself (but there are still areas that are spotty), but go outside the metro area and I'm LUCKY to get 2 bars on edge, but mostly 1 bar (which I can't use for data - haven't tried too much voice with the 1 bar) and no service. I work in southern maryland, and I literally count my blessings if anything will load whatsoever any further than 5-10 miles south of the beltway.
The lack of service is annoying, but it's worth it for the phone, honestly, because I really don't do much talking. I am waiting for the day I break down and "need" it and I don't have service though, that's going to piss me off. I actually thought about keeping a cheap verizon contract with my old phone just for emergencies, lol, it's that bad. I used to be the guy where everyone would say "I have no service" and I would ALWAYS proudly have service no matter where in the country I was. Seriously. Now I'm the idiot who has the "fancy phone" that "wow, that doesn't have turn-by-turn navigation? are you kidding?" and "so, your fancy phone doesn't have any service?" Heh.
But, back to your original question... I was on month-to-month on my verizon account for a while, and I just ported my number at the apple store, and evidently, porting your number automatically cancels your verizon account. (I called to cancel a few days later and they told me it was already canceled because I ported my number). So my advice is to wait until after your contract it up, don't say anything to verizon, go get your iphone, port your number, and it will automatically cancel your verizon account. You can call them after to confirm. If you are getting a new number, then just cancel your verizon account after you get your iphone. Hell you could get the iphone now if you want to do that, and just cancel the verizon service when that contract is up. You'd have two phones for a couple days, but who cares?