I have that same idea written down in a notebook and I was dying trying to figure out how to use RFID within a certain distance and always get the right action (open windows / put top down on a hot day) with only a card in your wallet or something.
On second thought, that's a much better idea, I vote for that one since SO many people would want it. Im not even willing to start tackling that project because right now theres only a bunch of small projects that can possibly be used to make this, but theres no central one. Confused? I mean like, there's bluetooth / RFID readers that are independant, then there's antennas, then there's card reader/writers, then theres bluetooth hardware stuff, then theres the actual program. All of these separate cost a lot of cashola.