I did a distro upgrade on dop to Ubuntu 13.10 Saucy Salamander. Maybe this time I will remember to re-enable third party sources, but if my record so far is any indication, probably not.

After rebooting it appears the login screen is again nicely spread out over my 2 monitors, instead of having a weirdly letterboxed screen that was mirrored on both monitors.

After logging in, I noticed the keyboard wasn't working in any application other than Unity Dash, but fortunately this is a well documented issue and is solvable by setting the "keyboard input method system" to "none", per this thread.