My Mom's new car (a Lexus LS400 -- ugh) has a couple of nice features. One, the key is usually used via some sort of wireless interface. It just needs to be near the car in order unlock the doors and start the engine, and no buttons on the key need to be pushed. Two, the built-in cell phone attaches to her cell phone via Bluetooth (or would if she got that option, which she didn't). I'm pretty sure that it actually uses the phone as the transmitter/receiver, as it can't do SIM cloning over Bluetooth (or whatever it's called) as it supports non-SIM phones/networks.

What this means is that at least one car company thinks that Bluetooth is a practical in-car solution and that it could conceivably be used as a key. (Even though I don't think it's the technology used for the Lexus key, it could be.) So there's no good reason a Bluetooth-enabled PDA or something couldn't be used to activate a car. Of course, you'd still need a physical backup key in case power failed.