I've read the computer controlled slightly rich mixture break-in system is also implemented in other cars, like the smart diesel. In gasoline engines it's quite possible they retard the timing a bit as well to put less stress on the engine as it breaks in, and in the Prius this was a significant jump in FE, not a gradual climb as one would expect from a traditional physical-only break in.

Also, as far as FE goes in the Prius, you will take a huge "hit" if your trips are short. The during warm-up time the engine won't shut off when the car is stopped, plus it's running a rich mixture to get the heat flowing into the catalytic converter, so there's a huge mileage penalty during the first 5 minutes or so of driving that really shows up with shorter trips but is masked if you make longer trips.

2001 Prius Photos       2005 Prius Photos
1992 Mitsubishi Delica Diesel-WVO Hybrid Photos