I'm a musician, too, and I think there's a lot of potential for a device like the N8x0.

Metronome, tuner (based on mic input), and tone generator could all be handy. It could also come in handy as a remote control for software like Ardour.

There's a drum machine for the iphone, but it's definitely more of a novelty than a serious or useful musical instrument. Still, I think a touch screen is potentially a very powerful thing.

If nothing else, maemo recorder is handy for getting a quick, if not good, recording of something.

BTW, Ardour is a great piece of software.