Hi,
I had the same problem. In my case, it was caused by using Xinerama together with nvidia proprietary drivers. I don't know whether this applies to Stephen's problem, but it might help someone else.
The solution was to replace Xinerama with the 'TwinView' option. Here is a good description on how to do this (second message): http://ubuntuforums.org/archive/index.php/t-75149.html [1]
I should also mention that I'm using Fedora Core 5 and that the problem occured only after upgrading the kernel to version 2.6.20-1.2316.fc5.
Cheers, Cristian
Links: ------ [1] http://ubuntuforums.org/archive/index.php/t-75149.html