This capability was confirmed way back at E3.  I was just rewatching the Wii music part of the conference to see how bad it really was. The horn guys kind of fell apart halfway through.  It's almost like they lost their connection or something.

This seems like a game I might try some time

Any of you know if there's a mode in this game, similar to the mode on Kareoke machines where you get a score depending how well you perform at the end? Like, immitating a certain theme given to you.

No, Wii Music has no method of the software scoring your performance in the "main" part of the game. The only scoring that takes place is the score you give yourself (on a 100-point scale) for an arrangement you create yourself.

The three mini-games, however, do have an automated scoring system, very similar to that found in Wii Fit. I have completed all levels of each mini-game, which has allowed me to unlock all instruments, performance stages, and songs. As far as I can tell, unlocking these items is dependent only upon completing the mini-games, not on the scores you get in those mini-games (as long as that score is above 0, of course). I cannot see much motivation in trying to improve my scores in the mini-games (it seems I have everything unlocked) other than simply trying to get better scores.

I finally got to play the game last week at a eGames 08 show in Melbourne (Australia..that's where I live) and sorry to say it played too simple for me. So I bought Guitar Hero WT instead!! Ironically in Australia Wii Music debuted at Number 1 ahead of the PS3 and 360 version of COD:WaW so it certainly don't need me!