I've just completed a Ubuntu 12.10 server build on a HP XW8400 desktop with 2 x GeForce dual head DVI gphx cards.
On the build bench I had 1 monitor connected & worked perfectly. 1920x1200 perfect I then plugged in a 2nd monitor into the 2nd DVI connector on Gphx card 1 and that worked 1920x1200 perfect.
When I connect a 3rd monitor into the 1st DVI connector on Gphx card 2, a mess of black (grey) and white pixels is displayed. no "X" from mouse movements is seen on the screen. The monitor works OK as I swapped it with one of the functioning ports on Gphx card 1 and it displayed perfectly. Monitors are all good.
All monitors are HP LP2475w's connecting with DVI ports to identical GeForce cards
I also have a 4th monitor which I'd like to connect but want to see monitor # 3 operating correctly 1st.
No alterations have been made to the "ubuntu-desktop" standard install available with Tasksel. Compositing is working fine on the 2 monitors also... Its actually a really sweet OS and the machine is running Very Very FAST!!!! I'm dead impressed with 12.10.