Apple also sells a car/air charger as well (which I own).
http://store.apple.com/us/product/MB441Z/A
First off that does not charge the battery on the computer and then there is the fact that it does not work on a lot of cars.
There's a reason it says Airline adapter and the DC adapter has an aircraft logo on it. If this was car compatible, Apple would market it as such and charge about 3x as much.