According to a rumour (from a German Nintendo employee), it's delayed here in Europe till Q1 2008, with the US getting Mario Kart Wii late instead. And since the Wii has region locks... I'll be pretty pissed if this is true.
Anyway, I'm getting it. The online play is likely to be amazing. Today's update confirms Donkey Kong as being playable (as if they'd actually leave him out) and a 15-second movie showing some snazzy fighting. Nice.