We've been informed that the mention of Wii compatibility on the packaging for the standalone Rock Band microphone was an error. A spokesperson from manufacturer PDP contacted Joystiq to say, "Our packaging mistakenly states that the microphone is compatible with the Wii. Unfortunately, this is not the case and will be corrected in future versions of the packaging. We apologize for the confusion."
The statement's timing seems odd following yesterday's confirmation that Rock Band is coming to the Wii. It seems there's some issue with this specific microphone, meaning it should not be compatible with Boogie or High School Musical either, both of which use microphones plugged into the Wii's USB ports.