Dubious:  adjective

1. hesitating or doubting

2. not to be relied upon, suspect

I think Matthew was succinct in basically saying that it is a 'concept' that should not be relied upon without being abrupt or rude.  And he effectively stated that thse shift's veracity is irrelevent.  He seems not to care about the statistical aspects of the name "Six Sigma", which has reached the level of Kleenex as a 'generic name' for Quality Improvement philosophies, methods and tools.  Most people* have simply moved on from the whole 'six sigma 1.5 sigma shift' thing.  As Matthew points out, it isn't relevent to improving quality, so it isn't worth the time to discuss it or attempt to prove or disprove it again (and again and again and again).  Life is just too short to get upset about a side issue. 

I've attended the last several years of the ASQ Lean Six Sigma Conference and I didn't hear or see a single reference to the shift.  People were simply focused on finding practical approaches that would work for them.  If we are going to champion Quality Improvements perhaps we are better served by focusing our efforts on the real issues we face, such as understanding the difference between enumerative and analytical statistics and diagnostics strategies that provide alternatives to fishbone diagrams and 'brainstorming'.   Let's give people practical tools and methods that they can use instead of telling them the old mythical horse is dead...

*the only exception being companies in India who seem to have just discovered the old writings but they too will find their way out of the statistical desert...Some people won't get it of course, but we'll never change their minds so I choose to focus on what I can do...