Well, I read about it on Sheldon's site and tonight I experienced it first-hand.
Hit a bump and my rear wheel got whacked out-of-line, chain came flying off for a few seconds and then quickly wrapped itself around my rear cog/hub. I was going about 20-25 mph and skidded to a stop in about 10 seconds.
Could have been worse since I wasn't in traffic and not going downhill -- glad to have a front brake, though.
If you haven't read the info on Sheldon's site check it out -- I did last week and now I'm glad I did.