Included in the flood of leaks that sprung earlier today was a flash video showing gameplay in Ultimate Band, a no-peripheral music game due out from Disney for the Wii by the end of the year. (The video did not address proposed versions for the PS2 or DS.)
Watching through it a couple of times, this is going to be far more casual than the gameplay in Rock Band or Guitar Hero. The instrument actions seem not to have any connection to the soundtrack, which plays throughout, with vocals. The songs in the video were familiar but all were covered ("My Generation" by The Who, "Fell In Love With a Girl" by the White Stripes, and "Our Time Now" by Plain White T's, all sung by what sounded like the same female vocalist), and the performers were cartoon avatars rather than motion-capped actiors.
First blush, with Ultimate Band, this is not a musical performance simulator. It seems more of a party game. You're moving the Wiimote and Nunchuk in was that intuitively resemble instrument play, but you're not hitting every note that comes over the speakers.
For example, the lead guitar "notes" passed through a side scrolling fret but had no effect on the music at all. An extended fretboard solo involved passing the Nunchuk back and forth rapidly, as if up and down a guitar neck while, I assume, holding the proper key on the Wiimote.
The bass guitar works the same except the notes are even slower to pass. It looked boring, to be honest. The frontman mode involves not singing but animating the lead singer through a variety of points-scoring postures and motions, including the "flourish" and "grandstanding", where players respond to on-screen cues to move the two controllers, and these in turn jack up a "crowd" meter that seems to have a multiplier effect on your points.
Of the four performers, the drummer looks like the fastest paced, and plays closest in time with the music. The difficulty in the others' music performance is minigame quality at best, and a focus seems to be on being able to enjoy the animations and socialize with other players rather than any approximation of a true-to-life-performance.