lmms seems a bit too cpu hungry on my pentium 4 ubuntu box so I have my doubts about it being usable on anything slower than that.

FLStudio 7 running under wine seems to be much more usable!

Can the n900 run wine?

also - can I use an external usb audio interface with something like an n900 ...

If the n900 can use an external audio interface and some usable dj/audio software
(thinking about mixxx, etc) I could get tempted to get one!

Of course production software would be nice too but even just usable dj software on such a portable device would tempt me.
(but I MUST be able plug in extra audio interface for that to be any real use - would need headphone and main audio out at the same time!)