This is a good thread. Lots of interesting theories. As others have stated, it's probably a combination of factors.
But one thing that makes this spate of fast times seem even more remarkable to me, is that the average high school kid is probably more sedentary and out of shape than he has ever been. I realize we're talking about elite athletes, but I would have thought that the potential talent pool would have been diluted significantly by so many kids being such couch potatoes.