General Cycling Discussion - What type of bike would be good for a ride across the united states?

I've never been a bike enthusiast, but this summer i'm going on a ride across the united states. I don't know much about bikes, but suddenly need one. What would you recommend? should it be new or used? i don't have more than $700 to spend on it. If i could get a new 2007 trek 1.5 or 2.1 in that price range should i, or should i find something on ebay?
If you can't tell i don't know a ton...please remember that when answering=)

What organization are you doing this ride with? Over how many days? Supported or fully loaded?

And you might want to ask this question in the Touring forum where you'll be able to get more specific answers and read other threads about long distance touring.

I'd take my Atlantis, with the lowest gears I could stick onto it. In your price range, though, I'd suggest looking for a used touring bike, something with triple chainrings and room for large tires (700x32 or 35, anyway). It has to fit, of course, and you don't want to dawdle in finding one--you need to be on the bike and riding as soon as you can (well, maybe you don't if you're 24, but I'd certainly need months to prepare for it). Putting miles on it now is also a good way to figure out how you like it set up, and it gives pieces a chance to break in, or just break, if they're going to.
Alternatively, there ARE some usable new bikes in that range. Someplace like REI would be a good place to start. My wife's K2 would make the trip, and I think she paid $450 for it on sale there. A mountain bike with road tires also wouldn't be a bad choice if you can get comfortable on it. I did my first three centuries on an old Mongoose, because that was the bike I had and I couldn't afford another. You may be a little slower, but what's 1mph on a 3000-mile trip?
There's been a lot written about touring, long distance cycling generally, and how to train for long rides. Check out as much as you can, because you're going to discover things about mile 80 of the fourth day that you don't even suspect now.

Its about 65 days. We have one support car, so we don't really need to carry anything. i'm already doing some training.

I'd suggest a Pilot if you're looking at Trek, though you'd probably have to go used to fit your budget. The Pilot bikes are slightly more comfort-oriented and suited to long days in the saddle.

Other bikes with this "comfort" geometry include the Specialized Roubaix, Felt Z line, Giant OCR, etc.

I concur with Peek the Geek, something light and sporty but with a more relaxed riding position. Make sure it has some lower gears , you dont have any real use for high racing gears but you will run out of low gears at some point.
Tyre size is a personal thing but many would prefer 25 for unladen, 28mm for a light day-touring load and 32 for a light hostelling load.
The most important factor is correct fit (www.peterwhitecycles.com/fitting.htm), closely followed by correct fit.

Surly Long Haul Trucker or Cross Check.