Here's a recent thread in Viewfinder with some good info: backdrops

Backdrops are usually a cloth of some sort (muslin, mostly) or seamless paper on a large roll. Never seen cardboard, and I'd probably stay from something too colorful - you'll likely get tired of it quickly. Muslins cost a few bucks, but fold up very small. A 10x20 can easily fit into a small duffel bag. They'll also last a long time if taken care of. Get the wrinkles out of it by misting it with really hot tap water. Get the rest of the wrinkles out by using a wider aperture ;) .

Paper is nice too but a full roll is about 9' long - won't fit into my VW too well. It's also not very durable (obviously). Half rolls are also common and both are pretty inexpensive. Might be good if the length isn't a problem and you want to experiment with colors. Depending on your lights, you could gel them too to make a white or grey background into whatever color you want.