Rock Band microphone peripheral's packaging says that it works on the Xbox 360, the Wii, and the PlayStation 3. Of course, there is currently no version of Rock Band for the Wii, nor has there been an announcement. Score Hero forum member Rocker74 picked up one of the mics and took the picture seen above. You can just see the word "Wii" if you kinda squint.
Did the packaging design for an accessory leak an unannounced game? Probably not. Our guess is that this USB microphone is compatible with the existing Wii singing games, which means High School Musical, American Idol, and EA's own Boogie. We would, of course, love to be wrong. We'd rather have the full Rock Band experience instead of just the expensive peripherals.