first of all, your completely wrong. the prius right now doesn't allow you to plug it in. the gasoline engine recharges the battery. and since it still uses gasoline, i don't care how efficient, it's still not good for the air.
even if you did get your electricity from coal, you would be cutting carbon footprint by a lot. there are some kits that allow you to plug in a prius and to allow it to only run on electricity. and according to studies, plug in hybrids use very little electricity. do you own a plasma tv? well, that plasma tv's probably using more electricity than the prius believe it or not.
the most ecofriendly you can get is a bike or walking.
but if your lazy like most of society, your best bet is public transportation or FULLY electric cars. with an electric car, you'll be cutting your carbon footprint by a lot. electric motors use less electricity that clothes driers. as i said, even if you get coal electricity, your cutting your footprint cause electric motors are very efficient, use little energy. plus, a gasoline car on average emits EIGHT TONNES of CO2 into the air.
also, obama's pledges to create cleaner electricity, so your cutting more CO2 there. but here's where you can score the biggest. if you put a couple solar panels on your roof, you can run your whole house completely off the grid. so basically, you don't pay for your electricity, and neither does the earth.