Larry Leach has an interesting article about the effects of combining the Theory Of Constraints with Lean and Six Sigma. The article is based on a study published in APICS Magazine May 2006.
I intended to do a writeup of the study of my own, but I plumb forgot, until Jack Vinson reminded me with his article. In his article, Jack links to an interesting article by Tom Davenport, Why Six Sigma is On the Downslope by Tom Davenport, but you will have to visit Jack's site to get the link.
Oh, there is one thing. While you are reading the article, consider this: what evidence does Tom Davenport present that Six Sigma is on the downslope? What assumptions about Six sigma popularity are readers likely to make after reading the article? Do these assumptions have a basis in fact?
As for Jack's comments on Davenport's article, I agree with them.