VBox-OSE is available from RPMFusion, although REV. 3.1.6 has not yet appeared in the repo (looks like 3.1.4 at the moment). Perhaps an update will be forthcoming soon.
VBox now has it's own YUM repo for the "standard" version, which includes some features not available in the OSE edition. I would suggest that you should consider installing the "standard" 3.1.6 RPM for F12.