The January 2005 issue of Scientific American has, in the Working Knowledge column, a nice technical explanation of how keyless entry systems work. There's even an explanation of how the system recovers if you press the unlock button on your fob before you're in range of the car's sensors (this, as it turns out, puts your fob and the car out of synch).
This was a good article-even explains that in Europe a different frequency is used. US and Japan use the same frequency. It goes over the immobilizer also.
Thanks for the pointer -- I'm curious, and will take a look. There's also a very good on-line description on the difficulties inherent in stealing a Prius.
