Why would you want to do this? It's very unusual, and I can't think of any practical reason for it - that's probably why you're having such a hard time finding ways to do it...
...however...
You could set up RealVNC server on the desktop, which would allow you remote access - if you went full screen mode I guess, essentially, you would be using your laptop as the monitor and the keyboard for your desktop. Shouldn't have much lag on a local network, so that should work out good.
How about that?
don't know if this is what you mean or if it will work??
http://www.epiphan.com/products/frame-grabbers/kvm2usb/?gclid=CLPWlej7u4oCFQImUAodVUsXPg
or this link
http://canne.wordpress.com/2007/03/22/laptop-as-vga-console/