Originally Posted by hvengel
I suspect that the reason that this is low on XOrgs list is because the open source commumity is in the process of transitioning away from xinerama to xrandr 1.2 and eventually xinerama will go away.
This is a common misconception. RandR 1.2 does not provide a replacement for Xinerama because it doesn't bind multiple X screens together into a single screen at the protocol level like the server's Xinerama implementation does. What it does do is provide so-called "Fakerama" information to clients about where the display devices are, similar to the how the NVIDIA driver provides information about TwinView displays. Just like TwinView, it currently requires a single GPU with a single framebuffer. For the most part, you can think of RandR 1.2 as being equivalent to TwinView.
There's been some brainstorming about how to tie multiple screens together in a Xinerama-like way with RandR, but nobody's made any concrete plans.