My fears were confirmed when I took my bike in for the guys at Outback Bikes to see it. The rear derailleur is toast. Apparently, the brand new Shimano chain snapped at the assembly link in such a way that the link end came through the chain drive path like a grappling hook and ripped by rear derailleur to shreds. They say they are going after Shimano for the warranty. I will need a new Deore XT rear derailleur and, obviously, a new chain, which will be a Sachs chain. Avoid Shimano chains. The service manager at the shop has actually seen this same thing before, and refuses to ride with Shimano chains. I think his own personal episode of being stranded by a Shimano chain may play into that.