The hurdle for this is that to do it well, you need polyphonic pitch detection, which you could say is a little tricky.

However, I worked on the prototype for a music app that does polyphonic pitch detection for a greater effect than page turning. It listens and follows along note by note as you play your (real) guitar and, as you hit the notes properly, actually uses the master tracks of rock songs to give you a very cool sensation of playing the real song. It's called Rock Prodigy: Guitar and is free with per-song IAPs if you want to try it out.