I suppose it could be possible to have a external box with a guitar plug IN and a usb to connect to N900, with a converter from analogic signals to digital ones. Then we need a software to precess the guitar signals. Probably it's more simple to use some pod or lines6 kind of thing...

Naturally it is a nice project.

It could be a nice things also just connect my v-amp2 to n900 with usb and could record on the n900. (the v-amp has not native usb out).

Well, I have not idea if the last thing is possible or not, nor how to do it.