I was seriously disappointed when I found that the ipad is not capable of turning the orientation of PDF files (like the ones available on the Werner Icking Music Archive or the Petrucci Library) from landscape to portrait. I also found that it is not possible to conveniently turn pages or even conveniently scroll through music.
I'm not holding my breath, but I'm secretly hoping that some very smart music-loving technical person will eventually develop a mac- and windows-friendly music-reader (wouldn't it be appropriate to call it a "notebook?") that would really work for musicians. It wouldn't have as large a potential buying "audience" as the ipad, but it would help a lot of musicians. This is what my machine would require:
1. A screen that can be viewed clearly under all lighting conditions, including strong stage lights. It would need to have a viewing area that would be at least 8.5 x 11. 9 x 12 would be better.
2. A button on the lower and/or upper right hand side of the machine that would function as a page-turning button. It would need to go in both directions to account for repeats.
3. A method for annotation (fingerings and bowings) on the downloaded copy (a stylus, perhaps), and the option to save an annotated copy in an easily-accessible format.
4. It would have to have a very smart and flexible filing system that could organize sheet music into categories: period, genre, instrumentation, etc.
5. It would have to be silent, like the ipad.
6. It would have to have the capacity to do e-mail and send attachments, so there would need to be a functional keyboard--either internal or external (I can't stand to type on the ipad touchscreen).
7. It would need to have a long battery life and would need to be easily recharged.
8. It would have to be sturdy, but it would have to be light enough to sit on a music stand.
9. It would have to be affordable for musicians.
10. Here's my pie-in-the-sky dream for such a machine: it would work as a scanner as well as a reader (hence the ideal larger screen size).