Mac Pro can't safe boot
This appears to be covered in detail (without an answer on the Apple discussion forums (thread 1 and thread 2).

The second link in particular goes on a long way, with rumours of a workaround (though not what the workaround might be), ways to disable fsck so that it boots (though what the point of a safe boot without the fsck is I'm not sure), that it may be related to the RAM installed on the Mac, and that it may be fixed in the 10.8.3 seeds.

Good luck until it clears up.