[ringgh0st]

You'll need Vista for the guest XBMC OS. Windows 7 might work too.
I have my database stored on the server. I also use media info plus for all my nfo and fanart generation. The server also runs XBMC but it's just to update the database.

Here's how to do it.
Server machine (or NAS housing the database) should share the database folder read/write.
Guest machines will have to symlink the database folder to the server. It only works with symlinks and not shortcuts. Google mklink.
Guest machines will have it's own userdata folder and usersettings so the resolution and settings will be independent for each guest.
Make sure only 1 machine is on at a time accessing the database. I'm not sure what would happen if you have multiple connections.
Also, I run the same XBMC build on all machines.

Issues - For some reason, after a few library updates (around 6), it doesn't want to update any more. For example, if I watch 6 shows, the 7th show won't be marked as watched. A guest computer reboot solves it. I'm sure I could write a small batch script that unlinks the database and relink it but I haven't gotten around to it.
If you ever uninstall a Guest XBMC, do not remove userdata otherwise your server database will be deleted. Found that out the hard way.
Initial database access has a little delay. When I go to TV shows, it takes about 10 seconds to list them all. I'm not sure if it's the shared database or the fact that I have 10000+ shows.

Benefits - I only have to update the database once. It's fast anyways since the server already has all the nfo and fanart. Shows that I watch in one room will be marked off in any room. I can stop a show in 1 room, and shutdown xbmc, go to another room, and resume where I left off.
I also share background art off the server. New background art I put will be availabe to all guest XBMCs.

AWESOME! Thats what i wanted and hoped to hear. thanx

@Neil. i read somethin about media companion but didnt get the point why i should use it. because xbmc does the same thing when updating the lib.

i'll try both ways and'll give feedback. thanx guys