I can..
..see this being sold to a decent gaming company for several million dollars. No, really. This would make an excellent PC game, no doubt. (Not so much with Flash though, but only because Flash doesn't necessarily run that smoothly on all computers.)
The graphics are beautiful, the music is great, and the controls.... Well, I can definitely tell you worked very hard on the note transposition as well as you can using only 8-10 keys. But it works! So no worries there.
I'm serious. I'd pay $10-$15 on a PC game like this. It's very addictive, I'm only just now realizing that I've been playing this for 3 hours, it's almost 9 in the morning and I need to get some sleep.
Add more songs! This is not a criticism. I've just played through all of them many many times.
Keep up the good work!!!