The Simpsons episode 705, "Lisa the Vegetarian", explains the very reason the little star girl Lisa became a vegetarian: a cute little lamb at the petting zoo, and she just could not bite another lamb chop. (I always wondered why Lisa was feeling a little out of the family...)
Apparently, this episode is one of the most favored shows aired on The Simpsons. MSNBC rated it the second most favored show of all Simpsons episodes. Even Wikipedia has a page dedicated to this episode, citing popular culture icons such as Beatles and Paul McCartney. Crazy!
I believe there are vegetarians in the Simpsons crew, which is pretty cool. One of the new episodes, "Apocalypse Cow", also shows Bart (gasp!) showing his love of a cow.