This Sparkfun project by Nate creates a keyless entry fob for a Mazda using a Nike+iPod kit, allowing you to leave your car keys at home. Using a serial board you can read the input and output bits of the footpod and grab signals and decode them.
He then used an Arduino Pro Mini connected to the lock and unlock wires in the car. Now all he has to do is walk up to the vehicle and it unlocks automatically.
While testing I found the range of the transmitter (the footpod) was really pretty impressive at 50+ feet. This was cool, but I didn’t want my car to be unlocking/lock while walking around my house or office. To cut down on the reception, I sabotaged the antenna. I could have hacked into the footpod, but then it wouldn’t have fit so nice in my pocket. Instead, I completely wrapped the receiver with a few layers of aluminum foil. This cut the reception down to 20-25ft. Jeesh! What do I have to do to kill the power on these things? These devices are designed and built very well!