I'm not going to tell you it's there if you say it's not, but I was denying mine had any just a couple of days ago, too. If I look REALLY closely I can see it. But I mean I have to stare at it, and I'm pretty sure it's almost non-existent right after turning the screen on after it's been off for a while (i.e. it's at least partially temporary).
I am blessed with perfect vision (wish my other senses were up to par, the pixel grid on my laptop panel drives me bonkers) and I am quite certain I do not have any burn-in whatsoever. I've even examined my screen with a jeweler's loupe while running a pixel test video fullscreen on youtube. In none of the primary colors do I have any indication at all of burn-in or image retention. None at all. End of story.
As others have said, I'm sure there are a few phones out there exhibiting this problem. There will always be lemons in a mass-produced good. Always. Maybe there was a bad batch of silicon. Maybe one of the manufacturing machines was operating outside of calibration specs. I don't know; I don't work in the factory.
What I do know is that perpetuating that this is a problem with Nexus 6 in general is becoming apparently more and more asinine. There is no evidence that this is a wide-spread problem. For every user reporting burn-in, there are many more reporting none. And of course, there's even many, many more who will have no problems, and we'll never hear from them.
If my screen burns, I'll gladly eat my shorts. I'll also gladly claim warranty service from Motorola, and happily accept a swap, fixed screen, or factory refurbished Nexus 6 if that time comes.
edit: and I'm mainly talking about brand-new, low usage time results. I think we all know and accept that the AMOLED will degrade over a period of time dependent on usage. That's not in question. The question is if these things are burning out-of-the-box. From what I've seen, I have to say, "no."