SEGA has revealed that a Wii version of Sonic Generations was going to be released, but development has been "abandoned" in order to keep the game in HD. Apparently SEGA's just too good for that grotty standard-def garbage.
"The primary goal for the console versions of Generations was reimagining the series' classic zones in vivid, high-definition graphics," said Takashi Iizuka in Nintendo Power. "The Wii hardware wasn't a good fit for that concept, so we abandoned development on that, and decided that for our Nintendo audience, we would make a fresh start with a Nintendo 3DS version of Generations."
The 3DS version sounds like it'll be similar to the DS version of Sonic Colors -- a fully 2D alternative to the console iteration. Frankly, Sonic's usually better on handhelds anyway, so I wouldn't cry too hard about this not hitting Wii.