Six Sigma is about eliminating process variation, but not just any kind of process variation. It's about eliminating "destructive" variation. Destructive variation is value-subtracting variation (rather than value-adding).Some go too far with Six Sigma and interpret it to mean that ALL process variation is "bad." I don't subscribe to that belief. I think there is intentional variation that adds value, as well as "essential" variation that is either unavoidable, or else emerges naturally (and beneficially) out of the creative & learning processes.
Many have tried for repeatability & reproducibility where it isnt always appropriate. The knowledge creating aspects of software development aren't always appropriate places for procedural repeatability and removing variation. The feedback/validating aspects often are. I think Software CM and testing are appropriate places for this: anything that is appropriate and desirable to automate (like building and testing) would be an appropriate for removing variation and SixSigma techniques.
So just like not all conflict is bad, not all variation is bad! And Six Sigma efforts should (I think) limit their focus to destructive variation for the portions of their processes where it make sense to have both repeatable procedures and reproducible results.
I think I'm actually starting to come up with a combination of Agile, Lean, TOC and Six Sigma that is both cohesive and coherent. I credit David Anderson with starting me down that path. (Look out! CMMI might be next :-)