IT seems to me, that system (if not whole, at least barebone version of it) would be needed to work as HDMI out to. Well, we need X on pi listening to export DISPLAY from out N900. It's possible that - using at least few GB SD card - Raspberry Pi with a system designed to do <whatever> would be also possible to act as HDMI out = universal thing to accompany our N900 + with possibility to work standalone for plentora of tasks.

/Estel

// Edit

As what we're planning to do, is to set up ARM full-fledged computer to act as X peer for HDMI output - no matter, that this actual computer is about size of credit card - I think people with linuxboxes can start experimenting right now. Any linuxbox with USB port and VGA or DVI/HDMI should be able to do that. Starting to experiment right now, we can have plentora of already tweaked solutions just waiting for arrival of our Raspberry Pis.

(Especially, that OP here is far from being knowledgeable about X voodoo, so helping hands greatly appreciated).