http://www.avweb.com/news/sayagain/191072-1.html has a (now retired) US air traffic controller's thoughts on a fatal collision involving two of the three planes a European controller was responsible for. It suggests that not all cases of sleep deprived controllers are harmless.
Like most serious accidents, the sleep deprived controller is just one part of what went ridiculously wrong there. Had the controller actually been sleeping the entire time, and the pilots followed the automated systems, it's entirely possible that they would have been ok. In addition, the equipment failure/downtimes severely limited any operator's situational awareness.
It's one of the cruel ironies here, that a completely passed out controller likely would have lead to a better outcome.
In a macabre follow up to the crash, the (Danish) ATC on duty was murdered by a Russian man who had lost his wife and children in the crash. 3 years later the man returned free to Russia, was hailed as a hero and given the position of a deputy minister in North Ossetia.
