Linux Mint 14, Unity and dual monitors. Seems to work fairly well indeed.
This is what the Ubuntu Unity desktop looks like with a dual monitor configuration. The launcher bar is on both desktops; but the dash when clicked on either desktop is independent of the other. And the wallpaper will span both desktops. This is on Linux Mint 14 and it is clear to see that this does work pretty well after all. Unity is not the best desktop to run on Linux but from the looks of this it appears to function pretty well with a dual monitor setup. This is pretty interesting. I have the dual monitor configuration set up in the xorg.conf file and every desktop I have will adopt this configuration; but some work better than others. I installed Windows 7 Enterprise from scratch and after installing the display drivers I was able to configure the extended desktop very easily. Linux should come with its own simple GUI tool that can configure the dual monitor configuration and write an xorg.conf file with the proper configuration settings put in automatically. That would be very helpful when you are setting up a Linux distribution and you have multiple monitors. The ubuntu-tweak utility does not have any facilities for configuring multiple monitors with Unity though.
This is how I configured my dual screen setup on Linux Mint 14: http://www.securitronlinux.com/bejiitaswrath/linux-mint-14-dual-monitor-support-works-after-all-with-ati/. This configuration with the ATI hardware works perfectly. And here is how to install the ubuntu-tweak utility on Linux Mint 14 and Ubuntu 12.10: http://www.noobslab.com/2012/10/install-ubuntu-tweak-081-in-ubuntu.html.