What a cool idea. You'd get the best of both worlds, I could go to and from work and never hit the gasoline engine, but then when it's time to drive to Redmond, I'd get 100MPG on the freeway and hit the gas engine occasionally. If only it were $1,200, and not $12,000...from Mike's List:

A startup near Los Angeles, California, called Energy Control Systems Engineering has created a system for tricking a Toyota Prius into delivered up to 180 miles per gallon in the city, and up to 100 miles per gallon on the freeway. The system involves extra batteries, which must be charged by the user, and a hack that tricks the Prius into thinking its batteries always have a full charge. The company plans to sell the hack via a company called Clean-Tech for about $12,000.