Hi, I had the same exact issue using 13 iPads and a Bretford Powersync cart. On the first sync attempt, 3 of them had the "supervision failed" error you mentioned. It was the same 3 three every time. I just kept clicking refresh over and over until they finally synced without any errors.
However, after updating them all to iOS6, one of them is now failing again. Just to see what would happen, I clicked "Unsupervise" and it threw me a different error saying "The host is not paired with the device". Not sure why it would say that, because all of these iPads were originally set up on the same MacBook using Configurator.
I'll keep digging.