It could be that once the suggestion that there may be QA issues gets around, people test their purchases more rigorously.
I had heard this when i was looking to upgrade from my 17-85 IS to either a 24-70 f2.8 or 24-105 f4.0 IS. So when I had 28-70 (the fore runner to the 24-70) on apro - as well as taking shots out in the real world - which looked good - I did some test shots. Now it definitely had better contrast than my lens, but it was not as sharp. I ended up buying a 24-105 as this suited my type of photography better, but even then I had to take the first one back -
see this thread NB - Leif suggests at the end of the thread that Nikon QA is not that brilliant either
Either way it pays to check - after all you are paying for quality - make sure you get it