Maggie speaks! Until now, Marge and Homer Simpson's youngest kid hasn't hogged many big plot lines. She isn't moody like her big sister, Lisa. She isn't mercurial like her even bigger brother, Bart. She doesn't get around much-two or three shocked steps in her signature Sweet Pea sleeper and she's splat on the floor again. But that doesn't mean she's not a fully formed, complexly individual, admirably resilient infant woman-with a fan following of her own. Matt Groening's animated baby has all the wisdom of a person without teeth and all the blessed good luck of a cartoon invention who is always safe from harm. (In an early episode when she crawled away during a family camp-out, bears adopted her as their own.) She deserves some developmental attention. And this season is her moment: In December, Maggie Simpson will unplug her philosopher's pacifier and utter her first words, with a publicity buildup worthy of the late Greta Garbo herself. ''It's a major surprise guest voice, but we'll plant some clues leading up to the event,'' teases Groening, who does assure that the guest in question (Elizabeth Taylor? Madonna?) will be ID'd: ''No more uncredited voices. If you're there, you're going to have to admit it.'' Groening is one pleased dad about his littlest Simpson, who is modeled after the early years of his own younger sister, Maggie, right down to her pacifier and sleep suit (''Those things are designed to make babies fall down,'' he reports). ''The rest of the family-even Lisa-say things that incriminate themselves. But Maggie just looks on in silent vigilance,'' he says. Then he drops a satisfying little bombshell: Among this year's antics, Homer will meet God. We can't wait for Maggie to suck on that one.