It would be GREAT if you can buy electronic sheet music. I sing in two choirs and I carry large music portfolios to each rehearsal. It would be totally convenient if this service was available. With my sheet music in electronic format, I could enlarge sections that I'm practicing, I can easily skip around the pages. I can take maestro notes during rehearsal and maybe even use the iPad during the performance?
The ideal would be to record a rehearsal and then on my own time, sing along with the recording, learning the literature, and being able to perform fully prepared.
Check out "Classic Melody Book" in the iTunes store
I heard that they have a new app coming out soon that fits exactly what you need.