So, according to this article, his reasons are not completely eco-friendly, but at least Matt Groening's driving a Prius. Groening, creator of the Simpsons cartoon among others, likes the fact that he can sneak up on people in parking lots without them hearing his car. Sounds like there's a little bit of Bart in him, huh? As a matter of fact, he nearly named Bart's character after himself.
I also noted in the article that the Simpsons movie is going to have a green theme. What exactly that means is still unknown, other than the fact that Homer nearly causes his entire town of Springfield to be evacuated due to toxic pollution of its' river. Oh well, whatever his reasons, maybe by driving a Prius, fans of the show will be motivated to go green themselves.
[Source: Times Online]