This errata?
http://download.nvidia.com/XFree86/L...a-1_0-4363.txt
That one doesn't say anything about Xinerama...
For the problem, it sounds like the games that are running aren't Xinerama-aware, or something... Or maybe the window manager doesn't work well with Xinerama? Well, no, the WM shouldn't matter much. Hmm...