Comment 119 for bug 742544

Joost, please run this command to confirm you are running the PPA version and to gather useful info:
    grep UScreen ~/.xsession-errors
And attach the output of the command here.

With drivers that have proper xrandr (all except NVIDIA), the preferred way of changing the primary screen is in the Gnome Displays control panel. Drag the little black bar (at the top of one of the screens) to the screen you want to be primary.