Actually the creators of the Simpsons gave him a birthday, and it's in early May, so he would be a Taurus. I heard that on Countdown with Keith Olbermann one night a few weeks back. However, his birthday was on a different day in some episode, I think, so there is a conflict. Eh. He strikes me as a Taurus, coming from a Gem's prespective - kind of slow, kind of dense, kind of boring, and way too obsessed with material things (food, beer, sleeping...although love of sleeping is Piscean trait too, and I admit I'm taken with it myself, being a Pisces moon and all).
Don't any Taureans be too offended by that - I know that's just stereotyping and stereotyping doesn't really hold up in real life.
I suspect Marge is a Cancer, or at least she's very stereotypically Cancerian - always fussing, fretting, nagging and with a strong domestic bent.
I'd peg Bart as an Aries and Lisa as a Libra - polar opposites, yet complimentary. Bart's love of pratical jokes, smart mouth and defiant nature is very Arian, while Lisa's artistic, cerebral and often waffling nature is classic Libran.