I think by next year, anyone interested in a Wii will know what Wii Sports is, meaning that Nintendo could un-bundle, and nearly all new Wii owners would still buy it, barring a better version getting released. The same thing happened with Tetris on the Game Boy, with Super Mario Bros. on the NES, and with Super Mario World on the SNES. Once the mass consumers know what a the pack-in game is, and what it does, it no longer has to be packed in.
Sonic 2 on the Genesis is another example. Dropping the game from the bundle and dropping the Wii's price the value of the game would earn the same money, but have a higher perceived value.