MusicReader is now also available for the Apple iPad:
http://www.musicreader.net/software/musicreader/apple-ipad.html
Looks nice, but to use your own music you need to spend $59 on the desktop version of the software. I may download the trial version just to see what it's like, but I don't think the iPad's screen is really ideal for this. I tried a different app called "Sheet Music Mobile" that will let you use your own files, but it actually practice at my piano, it just wasn't all that practical. When a full page is zoomed out, the music is really too small at a normal viewing distance if you put the ipad on your piano's music stand. If you zoom in, then you have the problem of scrolling around while you play.
Personally, if you were going to use this a lot, you'd want a full page sized screen...