Not Prius-specific, but Wired has an interesting article about RFID-equipped cars being stolen, and the trouble owners are given by insurance companies who doubt that the "unstealable" cars were actually stolen. Interesting point: that some transponder-equipped cars came with a secret "cheat" code designed to allow people who lose their keys to drive back to the shop. Is the Prius so equipped?
I wonder where the backdoor is on a Prius. There must be one, not everyone remembers to leave their FOB when they drop it off for service.
I would never have guessed the 2003 Honda Civic Hybrid would have such a backdoor... it does make me wonder whether the Prius has one. According to the article, with the Civic there is a way you can bypass the transponder and start the car by doing a certain sequence of emergency brake actions. To get the sequence that works for your car you would have to give the VIN to a dealer (or someone who has access to dealer computer systems). I would guess (hope) that Toyota wouldn't have left such a backdoor in, since with the Prius you don't even need a mechanical key to get in. So having such a backdoor would leave the Prius more exposed than the Civic.