everything depends on the car, as its been mentioned infra red locking on older models were so basic it'd be easily doable. new models use rf and encryption precisely to stop this sort of thing. if you're willing to pay to have the hardware mods done to your car you can basically pick how you want it to communicate with your n900 (i'd pick something that would work on different phones too for resale value and when you replace your n900 - ergo avoid ir and rf!)