I hate to be a downer, but the amount of power solar cells provide is a couple orders of magnitude off from what you need to power a vehicle. Think charging your iPod, not your Prius.

A bit of envelope math...

So a good days' charge will provide you with a little less than 8 minutes' worth of freeway travel. Much less if you're not using cruise control @ the speed limit.

I haven't checked into it, but I think a car needs more than 10 horsepower to maintain a highway speed.