This interests me also.

I have been thinking of how to implement a larger tablet-sized screen as a car system operating from the N900 to use its maps, music, phone, browser, etc. That would mean that the "car" system would not require its own SIM for internet access, and it begins to make more sense to remote the screen and control from the N900 which has all the personal programs and data you carry around, anyway.

At first the idea seemed to point to using a tablet with WiFi to VNC into the N900, but that would require setting up the N900 as a WiFi hotspot, and if already playing music and possibly using the FM transmitter, that's a lot of power drain even while charging.

So a USB tether from the N900 to a larger dashmount remote touchscreen would seem to make more sense.

It would probably be best if the larger "remote" screen had two pages: one for the remote of the N900 screen, and the other with local functions such as perhaps DVD player, OBD-2 data display, etc.