My friend is going to miss a dance he already rented a tux for because it is on the release date. Me and every person I know who has a wii has it on reserve/plans on buying it w/in the week it comes out.
I personally don't see SSBM having the same impact as Halo 3. Gamers know about the game, but they only make up some of the Wii market (while they make up all of the XBox market). The game will do well, but consider the marketing blitz Microsoft put behind halo 3.
Personally i see sales mimicing Super Mario Galaxy.