Coming at you from Hybrid Fest 2007, here's an interview with Ryan Fulcher about his plug-in Prius. Interestingly, Ryan does not consider himself a hypermiler - he doesn't like to worry about his driving habits, and just drives safely. While that may make those of you in the forest green spectrum scoff, the fact that Ryan is a paler shade of green really only makes his Prius' mpg figure more impressive.
He uses a battery charger from Manzanita Micro to not only charge the batteries when plugged in at home, but to have the additional lead-acid batteries charge the stock battery, extending the range of the Prius' electric motor. A simple solution, but unfortunately a costly one. A conversion kit can cost $10-12,000, which would take years to return in fuel savings. However, the point is not to make a business out of this kit. Admirably, it's to show the major automakers that plug-in hybrids are easy to build and that there is a market for them. One would hope they already realize that, but as Ryan says, this "proof of concept" should be pretty good evidence in favor of production.