I think overtraining is over rated and most people who use it, use it to slack on the training or diet.
I think there are certain factors that can lead to what one can call overtraining, but not might be completely derived from training itself. For example, if someone has a poor diet and is not getting enough sleep, then after blasting away for weeks on end, they are going to reach a point of neurological fatigue. Physically, they may be able to handle the workload, but their nervous system can no longer support the intensity required to train at a high level.
To overtrain, you would have to place a significant amount of repeated stress on your body so that it is not able to recover from the stimulus provided. Even if your nutrition is on point, if you are training the same muscle with a high frequency and with a high level of intensity, the rebuilding process within the muscle will not be complete before training that muscle and you will reach a point where you can longer gain size or strength.
However, a logical, well designed training program would alleviate this concern and is the reason such programs like DC and Mountain Dog training have built in cruises or periods of reduced intensity. They allow the body to re-set physically and neurologically so that one can make continuous progress.