Sheet music is truly one killer app for the iPad. I had never thought of it before seeing this app, but even if it's something as simple as scanned .PDFs of your sheet music, imagine not having to carry around backpacks full of books.
I have a lot of jazz fake books that are literally monstrous to carry around.
The iPad's the right size (ever balanced a laptop on its side on a piano's music desk?), and a touchscreen would certainly be convenient for turning pages and annotating scores.
Personally, I'd rather see that app on the Kindle DX, or a device with a similar screen. My eyes suffer enough already from all their computer time.
(If the app were on an Android device such as the Nook, I'd turn the pages with voice commands.)
I have a lot of jazz fake books that are literally monstrous to carry around.