Whatever you get, be ready to use it well the first couple of weeks, so you can make sure it'll work for you before you're locked in. Most will have a two week period where you can use the phone and return it without paying an early contract termination fee...some are 30 days (i think Cingular is)
I'd go for Cingular or T-Mobile, just because I like the way GSM phones work. They use a SIM card that stores your acct info, and you can store your phone numbers on it to. At that point, switching to another phone is as simple as removing the SIM card and placing it in another phone.
I'm using T-Mobile, which I switched to because I worked for them for about two years, and was pretty impressed with their attitude toward customer service. They went from pretty crappy to just being announced as the top of the customer service heap by JD Power for the second year in a row. That doesn't mean wonderful though, as it's all mass-market customer service no matter who you go with, and with thousands of reps and some outsourcing, you're never guaranteed a good experience.
Their plans are generally very competitive, and all but one regional plan give you free Nationwide calling and roaming. If you go into a dealer, or call them at 800-937-8997 and give them your home address / work address / whatever, they can check their coverage map to give you a pretty good idea what your coverage should be. They also have a cool feature where you can check the minutes right on your phone by dialing #646#...really nice for tracking your minutes. Others might have that kind of feature too, but I dunno.
If you go with T-Mobile, stay away from the Motorola phones. I've had wonderful Moto phones in the past, specifically the StarTAC, but most of the phones they supply T-Mobile are lacking in some way or another, or just outright junk. Samsung makes pretty nice flip phones, no matter who you go with.
Last nice thing about T-Mobile is the one year contracts. They don't even have two year contracts.
Whoever you go with, I'd recommend starting the service and getting the phone directly from the company. Go to a company store (prefferably...and get a business card for the sales rep) or use their web site or 800 #. That way you're dealing with just one company. When you buy from a third party vendor, you are often entering into a contract with them AND the service provider, and if things go wrong and you have to cancel, you could be looking at two early termination fees.
Finally, I'll repeat this because it's important. Be ready to use the phone, and take it to areas you frequent...just give it a good workout before you're locked in.
