What a coincidence! I just came back from seeing the vet and was asking her about this very thing!
I have a six-hour drive ahead of me on Friday with my cat. My vet recommended no food, but plenty of water. Also, I plan to have the litter tray on the floor of the car so he can use it whenever he needs. I'm sure I'll be stopping at every services along the way to scoop and dispose of said scoopage!
I don't plan to keep my cat in his carrier. He hates it, and actually enjoys the car. He generally looks out of the window for a while, and then curls up on the seat and goes to sleep. Of course, I've never done six hours with him, but he's been in the car for close to three hours before without a problem.
I'd say that whether or not you keep your cat in his/her carrier really depends on the cat. I agree with Meg on this one. If the cat is likely to get freaked out by it all, then you should probably use the carrier.
When do you go, Elynor? I'll be thinking of you! And please cross your fingers for me on Friday!