hi jack,
I am also into music creation, and I also want to use my N800 for some fiddling around. My solution so far is hardware-based:
I cut off the wires of the headset which is deliverd with the N800 (as I could not find this special 4-pole-jack-plug anywhere), tested which wire is the microfon input, soldered two resistors to it, and now I have kind of a LINE input. Problem of course: I do damp down the line to mic levels (in order not to overdrive the N800 input), then its boosted again, so Noise IS a problem...

I would be interested to find out about music applications (TRACKERS) for the N800, if you have any tips..???

cheers, karl.