In some states, such as California, it is illegal to have a laptop in the front seat while the vehicle is moving. This law was obviously created in order to prevent people from getting into accidents because they were looking at a computer.
Realistically, nobody is going to pull you over unless you are actually looking at it. A good idea is to go wardriving with a friend so he can hold the laptop and monitor it. If you must go wardriving solo, then put the laptop under the seat or close the lid.