trying to compile banshee (which was complaining about missing gnome-autogen.sh), i noticed the gnome-common ebuild had not been updated since gnome 2.8. To fix this, I copied the gnome-common-2.8.ebuild to gnome-common-2.12.ebuild, emerged it and compiled banshee. Hope that helps.
It must have fallen through the cracks... Fixed in CVS, thanks for reporting.