RedOctane's official website confirms the game's existence, target platforms, and a release window.
RedOctane, the original publisher of Guitar Hero and Guitar Hero 2 (and now a subsidiary of Activision), has confirmed on its official website that the next game in the series is planned for a wide multiplatform release, including a Wii version. The game is scheduled to ship later this year. Here's the exact quote:
"Guitar Hero 3 for the Playstation 2, PS3, Xbox 360 and Wii are currently scheduled for release in the United States in Fall of 2007."
No other details are given, but with this confirmation and the recently announced competition from EA's Rock Band (created by the original Guitar Hero developers, Harmonix), it probably won't be long before Activision spills the beans on GH3.
Thanks to rny9286 for the news tip!