A "Japanese source" that tipped off UK magazine Edge reckons it could be as a social networking MMO, and that we'll all (presumably) be able to travel freely to one another's towns in numbers, nabbing alien fruit and hooking rare furniture from Nookingtons; you would have to be extraordinarily angry with the world to resist such a prospect.
This is strictly a rumor for now, but Edge's solid past record on such matters makes it a tantalizing one. It also makes perfect sense, really, particularly as some of the core concepts of Animal Crossing -- community, trading, communication, and so forth -- are what MMOs are all about.
One does wonder, though: is the Wii ready for such a game? Animal Crossing: Wild World on the DS is notoriously temperamental when handling network play, and so far the Wii's own online capabilities haven't dazzled. We shall, as they say, see.