I have Cingular and just got the V400 which is pretty fun
Cingular is the only signal I can get here and so far I've had no trouble traveling with them. Once the buyout/merger with ATT is complete, I would assume they will have a huge network.
Consumer Reports did a report recently on all the major carriers and their phones. It's online at consumerreports.org. If you're not a member, LMK and I'll email the info to you