I just got my Dell laptop working with my new external flat-screen monitor as multi-head, more or less. I enjoy so much moving windows gradually from one screen to the other.
However, the Nvidia graphic cards don't seem to support the regular “Xinerama” way of doing multi-head. Instead they have TwinView, which I think makes X-Windows think that it has one big screen instead of 2 screens. I had to set this up manually in xorg.conf. I tried using the multihead options in Fedora's system-config-display, but that made xorg crash when starting.
I think that on a true multi-head system, secondary windows would open on the same screen as the application, but I think mine open wherever the cursor is. However, the GNOME panel is clever enough not to be wider than one screen.
And XINE gets confused. It squashes movies vertically, and I don't know how to stop that.
Note: In GTK speak, I might mean “display” when I say “screen” above. I get confused.