According to this piece, the best reason to buy a Toyota Prius is:
the feeling of satisfaction you will have as a responsible planet-dweller
I have not done the calculations yet, but my back of the envelope tells me that since the hybrids are making more efficient use of existing combustion engines without tapping into the grid, you DO actually get environmental improvements from it, but that still remains an open question. In any case this reminds me of an old survey I saw (I think from JD Powers) that found that "image" was the top reason people purchased a hybrid car.
There's a reason the Prius is designed to be so "eye-catching." Just purchasing a green product and not announcing it to the world is not exactly what some "E"nvironmentalists are going for.
Enjoy your weekend, I hope you have electricity so you can continue surfing the web!