The research I've read on Overtraining Syndrome refers to burnout just as an alternative name for the same thing. It's also been noted (both by researchers and a neurology consultant I saw) that the symptoms of OTS are almost a carbon copy of those of CFS/ME. It has been proposed that OTS/Burnout is the same condition as CFS/ME, just arrived at via a different route.
Just recently it's been discovered that chronic inflammation doubles the risk of depression and mood disorders, so considering that overtraining causes chronic inflammation, and 80% of OTS patients have depression, that doesn't surprise me. I think all these conditions are a lot more related than they first appear.
The problem with overtraining as I see it is that you can spend years piling straw on the camels back, and think there's nothing wrong right up until you put the last straw on.....