Just upgraded dop to Ubuntu 12.04 Precise Pangolin. The improved multi-monitor support is immediately apparent, because the login screen now displays something on both monitors, where before one was left black.
Unfortunately, now I have the Unity launcher on the left side of both of my monitors. It appears under System Settings > Displays, my monitor is detected as a single 3200x1200 display and Launch Placement is set to "All Displays". Switching that setting to "Laptop" (as the single display is apparently called), the Launcher is indeed displayed only on one monitor. Too bad it's on the left side of my right monitor.
Upon inspecting the NVIDIA X Server Settings, this does makes sense: it's configured to run as TwinView (which extends the desktop on GPU level) and my right monitor is the primary monitor. And indeed, I'm hardly the only one with this problem.
Turning off "Sticky edges" makes it a bit less annoying, but we're not quite there yet. Bizarrely enough, after making my left monitor primary, it's still doesn't get the Launcher. I'll leave both on for now and see what happens after a reboot.