Was just testing my mini keychain cam yesterday. It's very small and cheap (only paid 7,5 pounds from ebay, you'll need a mini-sd card as well though) and can easily be taped anywhere on the car. I fitted it on the back bumper for this vid.
Noisy coupe vid For the ones who are interested, I bought it from here:
ebay link I bought 2, so I can fit one on the front and rear at the same time. Should be nice for filming on Spa as well. You can record up to an hour on a 4gb sd card (in 720x576 resolution)