Update:
Sulu did some nice research on this topic, and proposed most efficient (up to date) approach - connect N900 and Raspberry Pi using so-called PC Suite mode, and mount N900 partitions (*every* partition we feel fancy) from Debian (or any other Linux-based distro we feel fancy, but Debian seems first choice due to Maemo being based on it, and EasyDebian) installed on Raspberry Pi. This also allow us to share N900's WiFi and cellular network, then either use programs bins from N900 itself (would require first-time special preparation on Raspberry Pi, but architecture is same, so should be doable), or just hassle-free 'clone' set of required programs installed on Pi.

This way, we would have (working simultaneously) N900 without *any* CPU/RAM extensive overhead, and programs working on our N900's files, run through Raspberry Pi and displayed on big screen. Of course, N900's partitions would be accessed simultaneously by both N900 and Pi, so we can even multitask to the point of having one thing displayed on big screen (lets say, movie, game, or photo slideshow active for kids/family), and another things running on our N900 (so, we can actually do productive things like hacking something in meantime ).

The only drawback is that we would not be able to use Maemo programs (as opposed to EasyDebian ones) this way (ho ever, there are some ideas for possible 'proper' workaround). Dirty workaround for that, is to use VNC with DisplayLink protocol used, as proposed by javispedro in his post:
http://talk.maemo.org/showpost.php?p...3&postcount=32

Of course, this whole approach mean that we can - if needed - actually benefit from MPU/RAM/etc resources of both N900 and Raspberry Pi, as totally exclusive multitasking, while still sharing the same storage/programs.
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Old update: (depreciated)
It seems, that it should be possible via USB networking with Raspberry Pi and X export DISPLAY: (using WiFi for that purpose should also work, but bandwidth may be insufficient fir higher resolutions). This approach should work with everything, except videos decoded in N900 DSP (bandwidth limitation of USB ~40 MB/s), or OpenGLES applications (some games like Neverball) - status of the latter is uncertain, and must be verified experimentally.

As for videos, workaround is trivial - play them via Raspberry Pi directly. They can even sit on N900 partition (most likely, vfat) connected to Raspberry Pi in Mass Storage mode. In any case, whatever mode we use, no hostmode on N900 is required (Raspbery Pi may act as host during USB networking), but it doesn't hurt also (if, for some reason, we prefer N900 to take role of host).

It's also possible to forward sound from N900 alongside DISPLAY (pulseaudio), so HDMI sound-transferring capabilities may be also used.

Particularly, we're planning to use full-fledged ARM linuxbox with HDMI out, to forward N900 display (of any set resolution) through it - it's all matter of fact, that, in our case, "full fledged linuxbox" is of credit card (thicker) size, and cost 25$ + shipping from UK.

Even more joyful fact is that Raspberry Pi, giving us long-awaited possibility of using our N900 with HDMI-out, isn't limited to "just" doing that - it may also act as USB-to-Ethernet adapter (in 35$ version, that contains LAN port), or any thing we would except linuxbox to do - in tandem with our N900, or acting standalone.

IMO, using it with our N900 is best "features" overkill in at least few years, comparable to USB hostmode (except, that hostmode required lotta hell of 'magic' difficulty level work - kudos to hostmode team).

Every interested one can contribute right now, even without Raspberry Pi in hand - requirements are only linuxbox with USB port and any video output (VGA, DVI, HDMI...). procedures for creating USB network + forwarding X Display over it should be same for "big" desktop computer and small Raspberry Pi, so we may create bulletproof procedure (or even scripts?) right now, to have it ready for Raspberry Pi (and final tweaking).
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