You're right chris but you still need a 3rd party which does support MIDI, and there is an additional step required. If you need to import and export a lot thats gonna piss you off eventually. But has its uses. Also for example on a live session while playing an instrument on a certain place of podium you could use the N900 to remotely control the laptop running some software. Maybe even with a simplified protocol, with the N900 having optimized touchscreen.

But all this is dreams and doesn't take into account the current state of Linux audio production which must be looked into. Not only available applications, also the infrastructure.

I'm not sure how well supported PulseAudio is, or how well suited it is replacement for JACK (low latency sound server), but back around 2003 we used JACK. For example, instead of an all-in-one application, one can use a patchbay for JACK to connect several (KISS) applications. PulseAudio also has low latency support (comes with performance price either way!), and networking support, but I don't think the latter is an alternative for MIDI. You must also use -rt kernel.

In fact, besides that allows you to learn applications and infrastructure running Ubuntu Studio on PC with a N900 remote connected, gives some idea of how the interface looks and works on N900 (but not performance).