Comment 7 for bug 423107

This really needs to be a preference. A lot of people like to use their laptop screen as the primary monitor, and then use the VGA for a projector display. Many people like to use a laptop at a desktop workstation, in which case the external display should be primary, because it's larger and better quality.

http://bugzilla.gnome.org/show_bug.cgi?id=562944 seems to be the solution here.