"There's a blank spot on my dashboard where the button is supposed to go," Pizer said. "I mean, the whole point of getting this kind of vehicle is supposed to be reducing our use of fossil fuels."

The fact that the feature isn't available in the U.S. may have to do with the way the Environmental Protection Agency measures fuel efficiency in the U.S., and that such a dual-power system would upset such measurements, said Coastal Electronics' Watson.

Kwong said Toyota doesn't offer the switch to electric mode because of U.S. laws mandating that it offer a minimum eight-year warranty for the car's power system. Thus, he said, by disabling the switch, the company is able to ensure a longer battery life.

Torrone said that he thinks Prius owners are likely to keep the hybrid car among the most popular vehicles for hacking for the foreseeable future.