Heck yeah! They already do this in some states. I got a Prius last November and love it. It doesn't get 65mpg or whatever is listed, as these things are tested in ideal (read: unrealistic) conditions. But I won't complain about 50mpg in a nice, zippy car. (Plus I can put the seats down, roll out my sleeping bag, and sleep comfortably in my "trunk" if need be...)
Hybrids
On interstate conditions at highway speeds, hybrids run exactly like every other car, exclusively off the gasoline engine. The are a little more fuel efficient because they have smaller motors, but they are lugging around the extra weight of the electric system. Hybrids are more efficient around town, not on the highway.