No simple adapter would increase the mpg by 30. Never heard of it and sounds way to good to be true.

It this rumor is true, it sounds like a "plug in" retrofit kit that can be installed on older Prius. Such kit already exits from 3rd part with cheapest being $2000 self install which simply adds more battery that can be charged up using household electricity. Expensive kit that allows pure electric driving for extended period of time could run up to $14000 per install.

Mike13433,
Toyota plans to begin testing the plug-in Prius (PHEV - Plugin Hybrid Electric Vehicle) in fleet vehicles in 2011. There are rumors that Toyota will release plugin Priuses to the general public for purchase beginning in 2012. A plugin version of the Prius could easily see an EPA rating of 80mpg.

http://priuschat.com/forums/prius-hybrid-news/64685-toyota-mass-produce-plug-in-hybrids-2012-a.html

I also own a 2008 Prius. The stock Toyota battery doesn't hold enough charge to make much difference with a simple addition of a charger. If you were hoping for a cheap way to improve the mileage, stick with hypermiling. If you're a real enthusiast, then check out the Hymotion.com website. I had the Hymotion L5 battery installed in my Prius last December, and it roughly doubles the gas mileage for as long as the charge lasts (about 30 miles). If your round-trip commute is less than 30 miles, you might be able to get close to 100MPG. Of course, it isn't cheap, but it really does work. Hopefully, as battery costs come down, Toyota will offer an option of a higher-capacity battery with a plug-in hybrid model. For existing customers, though, there is only after-market upgrades.