I've been running Ubuntu (10.04.1 LTS, 64-bit) for a while and just replaced my hardware with a faster machine with an ATI Radeon HD 5700 video card. I've got twin 1920 x 1080 displays. I downloaded the latest driver (ati-driver-installer-10-9-x86.x86_64.run) from the ATI web site and installed that.
I've gone through a few rounds of playing with /etc/X11/xorg.conf, and can't get things right. At the moment, it's in display mirroring mode, and I can't figure out how to get it out of mirror mode. If I run Monitor Preferences, there's a "Same image in all monitors" checkbox. If I uncheck that, the little preview window switches to show two monitors. When I click Apply, it asks me to log out and log back in again. When I do that, I'm right back to mirrored mode.
What's really weird is that I'm currently running a copy of xorg.conf from a coworker's machine. He's got identical hardware, and his display works fine. So, I'm inclined to think there's something else going on other than the conf file.
Any ideas what might be wrong?