X forwarding via SSH needs the xauth package, yes. But a good old regular X connection works okay-ish.

On your desktop machine, get a terminal and disable access control by typing:

xhost +

On the N900, get a terminal and type

export DISPLAY=youripaddress:0

where youripaddress is the IP address of the desktop machine. Then have fun by seeing which N900 applications you can start on your desktop! Some have trouble because they go fullscreen and then assume that their window is 800x480 pixels. And I don't know why the funny palette on the Map application.

Still struggling to see any practical application of this!