I now know at least what the trigger was: I had upgraded cairo packages from raring on this quantal machine (see Bug #1073372), and steam ultimately depended on libcairo*:i386, and different versions of the same package cannot live on the same multiarch install. Still, software-center should have prevented the package from being installed, or at least warned that packages were about to be removed.
I now know at least what the trigger was: I had upgraded cairo packages from raring on this quantal machine (see Bug #1073372), and steam ultimately depended on libcairo*:i386, and different versions of the same package cannot live on the same multiarch install. Still, software-center should have prevented the package from being installed, or at least warned that packages were about to be removed.