I'm currently exploring the iPad in the lecture theatre/seminar room as a kind of white board or smart board. Quite simply what I'm doing is using a stylus to write on pdf files using an app from the Apple store called neu.Annotate which was free but their latest version is 69p! I plug the iPad into a data projector and can talk and demonstrate and also play the music at the same time as scrolling through the score. Here's an example:
This is early days of course. I've pitched this to a new JISC funding scheme called Elevator and you can watch a short demo film and find more information HERE.