Seems to be fixed in Arch Vanilla 3.14.0-3...
Worked around, not fixed. We fixed the bug that rootfs has FSID 0 - which is illegal and has the potential to break lots of userspace, apparently. This has the side-effect that rootfs now has FSID 1 again and switch_root recognizes it as rootfs. However, there is no guarantee that that id isn't greater than 1, making the bug resurface again. Dave's patch to util-linux is still needed.
The switch_root fix has now been applied upstream and in our util-linux package.