Well, here is my problem :

I have a videoprojector hanging from the ceiling and, rather than having cables running all around the house, I would like to put a linux box right under it and then to use a remote computer to control this box and to display things through the videoprojector.

Of course, ssh -Y won't get me anywhere, as thus, I would display on the controlling computer screen. I tried the export DISPLAY=w.x.y.z:0:0 solution (w.x.y.z being the "upper" bow IP adress), but no go. Is there any solution, or am I bound to buying a bluetooth keyboard and mouse ?

so you want the same X display on two seperate machines? The quick way is to enable desktop sharing on one machine and simply view from the other. this is a doddle using x11vnc or an integrated vnc module for xwindows. this mightn't be as elegant as you want though, so i'd speculate that the dmx (distributed multihead X) program would let you genuinely create a clone on two screens across a network. personally i'd stick with vnc though.

Thanks. I'll have a look at dmx, which looks more like what I'm looking for, and else I'll go for vnc.

dmx did the trick, even better than I thought

So here is my "home cinema" install :

Server, plugged in the stereo, running the dmx server and doing the actual work.
Desktop, plugged in the videoprojector, used for wide screen display.
Laptop, used as a (big) remote.

Thanks a lot, I was able to give a new life to two old boxes this way.

Cool, enjoy.