Tom Poppendieck wrote (on the XP list):
There are three versions of SixSigma.
The original version dealt with reducing variation and had great value in cases in which product variation was the main problem. The emphasis on TestDrivenDevelopment in agile addresses this concern by eliminating unintended variation from design intent. When applied to design process, however, it was severely counterproductive.
To address this Six Sigma for Product Development added the voice of the customer to ensure that discrepancies between what the customer really wanted and what was developed was minimized. This improved value from the customer perspective but did not dramatically improve productivity.
Lean Six Sigma adds in addition a focus on FLOW and elimination of Complexity. The Lean Six Sigma movement is fully compatible with Agile software development.
Lean Six Sigma has 5 Laws
So you can demonstrate that Agile processes, done right, not only apply six sigma concepts, in their place, but go far beyond them to delight customers and to serve stakeholders with high productivity.
http://www.poppendieck.com/lean-six-sigma.htm has more.
Six Sigma IQ, also has more information about Lean Six Sigma http://www.sixsigmaiq.com