Any out there? Y'know, a sheet, notes, place notes on sheet, hear playback? It'd be convenient as heck.
Guitarist does something similar but not exactly what you're describing. You can enter in the tabs and play it by tapping a button (supposed to be like strumming or something). It's about the closest you can get to what you're talking about (as far as I know - and I have most of the musical apps).
Beatmaker is the closest you'll get right now, but it doesn't have a score editor like what you're asking for. I've been waiting for something like that since I got my iPhone. If Finale ever made an iPhone app, I'd freak out.