Though we had confirmation of some sort last year in Activision's report that Guitar Hero would be hitting 'all next-gen consoles', we now have information about the game's existence on Wii. No date is yet known, but here is the little article from the May issue of NGamer:
"Cult rhythm game Guitar Hero is on its way to Wii. It comes with a guitar-shaped controller and plays convincingly enough to make you think you're making real music. Hopefully we'll get the actual guitar, and not a pretend remote/nunchuk version."
They seemed to contradict themselves a bit there, but remember that they broke the news about Wii Twilight Princess.