Hyundai's Connectivity Concept, which won't actually go into production until around 2015, aims to harness the power of your NFC phone to make it way cooler than your keys ever were. Not only could the phone unlock your car and start it, but it could also save a sort of user profile, flipping to the right radio station or streaming music while adjusting the seat and mirrors for you as soon as you slam it into the dashboard dock.
Of course, as cool as that all is, there are security concerns. but issues like having phone theft rapidly turn into car theft could (mostly) be avoided with a user PIN or other measures. Most other concerns are just about as valid for NFC keys as well. Still, there's something that just seems weird about unlocking, and moreover starting your car with your phone. It'll still be while until any sort of implementation, but would you feel comfortable giving that kind of power to your trusty handset? More here.