Viper already makes one for the iphone, maybe they will do one for the droid soon.
Along the same lines, it would be cool to be able to unlock your car with the Droid. I locked my keys in my car trunk once and then called 1800mercedes and they unlocked the car via satellite.
The tech is out there and in the car. Now, if only I was smart...
Any apps out there or in the near future that will replace a car starter/alarm remote??