Wired has a cool story [1] about an engineer at the National Renewable Energy Laboratory who is working on a prototype plug-in hybrid that can go 50 miles on a charge, gets 100MPG, and has a solar panel on the roof that will power the car for five miles.

The car is a converted Toyota Prius, with advanced lithium-ion batteries. It's charged using an external solar array. One of their areas of research is li-ion battery efficiency and thermal management.