Cingular has pretty decent coverage right now, especially out west after they aquired AT&T Wireless (and their associated cell towers).
Without trying to sound like a TV commercial for them, so far I've had good luck with their service. They have "rollover" (keep your minutes) and "family plan" for multiple phones, not to mention free mobile-to-mobile minutes. Now customer service, that's a whole different animal... 'nuff said.
Also, I try to stay away from any phones with the built-in camera-FM radio-flashlight-music-player-latest-gadget. Had bad luck in the past; instead I try to go for the no frills base model. Nokia has been good to me, but I suspect that's just brand preference.
The older analog and digital cell phones were in the 900 MHz+ range. This signal carried better than the current GSM 1300 MHz+ range. Unfortunately those older setups have pretty much been done away with. Cingular is GSM, Sprint/Nextel is PCS, and Verizon... I have no idea. Out of all of those choices, a GSM phone will allow you to travel internationally and use it, as a lot of other countries utilize GSM technology.
And from what I hear, Sprint/Nextel has the option for newer phones to talk to each other without going through a cell tower, a la very short range two-way radio. This might be handy in an emergency.
As far as cell towers go, most, if not all, are on emergency power setups (generator, batteries, etc.). But in the wake of a major disaster will probably be rendered useless, if not by the disaster itself, then by the system becoming overloaded.
If you get a chance, check out CNET for cell phone reviews:
www.cnet.comGood luck!