CNET has a story on outlining some of the hacks users have made to their hybrids. Some of the possible hacks include:
Hacking your Prius
"It's quite a few steps, and you have to turn the car on and off, and you have to get the sequence right, (but) it was a piece of cake," said Pizer. "It reminded me of beta testers. Beta testers in a game will have a 20-step sequence for finding an exploit, and they're bizarre, bizarre sets of steps. It was very much like that, and I loved the idea of people figuring it out."
BTW, a Toyota spokesman pointed out they took out the switch for all electric driving in the US in order to qualify for the eight year warranty on the car's power system. That, to me, indicates a big warning to anyone who wants to hack these systems for better fuel savings. You may save more on gas now, but wait until the bill comes due for a new battery that much sooner!