Okay, maybe I'm the only one who thinks this would be pretty fun. But it probably has educational applications, especially when it's so hard to get an actual orchestra together to practice with. When I had to take conducting as an undergrad, we practiced with whatever instruments happened to be in the class. There was a pianist, a couple violins, maybe a clarinet, like three singers doing the viola part or whatever needed filling in, a cello, a flute, and a percussionist. It was pretty sad (though really, the saddest thing was practicing choral conducting with this same group of people singing). A conducting simulation would let you get down with an entire orchestra.
You may have this idea for free, since it likely won't make you any money.
ETA: Well damn. I just got this email from my sister Em:
My friend David reads your blog...I don't know why he couldn't just post the comment himself but I am forwarding it to you now.Maybe if I used a little thing called Google I could have figured that out myself. The game lets you play 60 instruments and conduct, and has ear training games and all sorts of things. Go Nintendo!
"You should let her know Wii Music just came out this week and does exactly what she is proposing: