I think it will have some interesting use of WiiConnect24, but not be a full-fledged MMO.
There could be a worldwide trading post for player-created buildings and furniture...
There could be a junior mode where you live as a resident in a friend's town...
Maybe they could even let 2-5 players build a single town together--stored on everyone's Wii, with updates uploaded to everyone's Wii every time someone plays.
But a full-fledged MMO, with, like, strangers talking to each other? Don't see it happening.
"[Our former customers] are unable to find software which they WANT to play."
"The way to solve this problem lies in how to communicate what kind of games [they CAN play]."
Satoru Iwata, Nintendo President. Only slightly paraphrased.