Cool idea: the new Toyota Prius hybrid car uses a transponder instead of a key. This means when you get within a few feet of the car the doors unlock. Furthermore, once inside, the drive system computer senses the transponder and all you do to start the car is push a button. No key.

I'm wondering if this uses an RFID chip in the transponder fob? A passive RFID, like used in Mobile Speedpass fobs, would mean the car is transmitting a signal and waiting for a fob to answer (the fob gets enough power from the transmitter to send out a reply). The other possibility is that the transponder is always broadcasting and the car always listening, but that would tend to wear down the fob batteries.

It's been a while since I've read up on RFID but it looks like you could hack together a keyless door entry system for a hundred bucks or so, a couple hundred if you get a devkit. You have a variety of choices for the fob: capsules, disks, even the dreaded ID cards. Insert the RFID capsule under your skin and you'd never lose your key again, although it might be tough to loan someone your car. This might hurt a little...

Hmm, come to think of it you could put RFID readers in your car and house, all of them keyed to quietly and automatically unlock whenever you approach.

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