I travel all over the country for work and one thing I can say for certain is that no carrier does a good job everywhere.
That said, here's my breakdown:
Pretty good coverage and great high speed data = Verizon
but, you pay a premium if you ask me.
Better all around coverage and so so data speeds (at least for now) = Cingular.
TMobile, Sprint, and the rest have not met the tests on my team of active travellers.
I currently use Cingular and I'm pretty happy with them - sometime I have better coverage than my coworkers with Verizon and sometimes they do. It's really hit or miss. Cingular is rolling out a new data network that should be great but right now Verizon seems to have them beat with data. I have a luxury situation, I have a Verizon card in my laptop that the company pays for so I get the best of both worlds I guess.
Shop them against eachother but I think either of them would suit your needs fine coast to coast. If your travel is more regional, then you may be able to narrow it down more.
Hope that helps. Good luck!