> granted, I'm not using an extension and X server from 1997 but something designed released in this millenium. ;-)
My 3-head system is running the current Xorg in Ubuntu 10.04, which still can't do XComposite (or XRandR) properly across multiple cards. It does at least do OpenGL, and windowed applications such as Google Earth are okay. Anything that tries to go full-screen can get very confused or worse yet trigger bugs in the X server, though.
It seems like every new release gets a little bit worse. For example with this latest Xorg the server chokes if you try to configure a 3-1-2 screen layout (so that screen 1 is in the center), which had worked fine for the past 4-5 years. The only person I've heard of managing to get XComposite to work across multiple cards had to use a several-years-old modified Xgl server -- it's never worked in Xorg.