I agree any muscle can be overtrained 100%, but very few reach the point where they overtrain. Depression starts to set in, people feel like shit in every way imaginable, its just not a continual feeling of being sore. There is a workout that was posted on t-nation where overtraining was the goal, full body 2x a day for 2 weeks, then you take one week of from everything. So I agree, but to actually over train a muscle unless you are like 60 years old or diabetic, you basically have to have that goal in mind. When I first started lifting I would train chest and biceps basically everyday for a week or two without taking a day off, still saw growth. Go ahead, try and overtrain your abs, you will find it very very hard to do. I hit mine everyday with weighted stuff and never gotten to point where depression sets in.
I heartily accept the motto, "That government is best which governs least"; and I should like to see it acted up to more rapidly and systematically.