UPDATE: Here is proof that the first version was not going according to plan: New GDR2 builds started hitting NaviFirm today.

Richard Hay over at Windows Observer has an excellent breakdown of the progress of the GDR2 rollout, which presently stands at 60% worldwide. I'm going to build on this information and offer my own view on what has happened.
I've seen the build in action on my wife's Lumia 1020, and was lusting over Nokia Pro Cam. For a while, I had complained about not getting the update, so I decided to flash the AT&T Canada developer ROM on my Lumia 920.
Now, it is important to understand that this build is already months old. Microsoft had to deliver this build earlier in the summer in order for Nokia to be able to deliver the Lumia 1020 on time. So Microsoft is already well beyond this version in the development process. In fact, they may have already wrapped on GDR3, as it has features that are required for Nokia's next major launch next month. The 1080p phablets due on Verizon and AT&T all run GDR3 and the Nokia firmware update called "Bittersweet Shimmer" (aka Orange).
Now, during the time my wife has owned this phone, I've noticed some issues she's had to deal with, like apps freezing in their tracks. My friend Robert Shubet (who also developed Hashtastic) had problems with his "Me Tile" updating consistently. And I had two separate bugs on the Lumia 920 that resulted in serious OS malfunctions while debugging applications.
This says to me that the build is buggy, and that Windows Phone is suffering from the lack of a beta program. So while the update has rolled out over half the planet, my theory is that AT&T and others will not roll out GDR2 if they don't have to, because it will cause more support issues than it will solve. Matthew Crommert is correct, Nokia, AT&T, and others will skip this release and pretend it never happened, just like they did with the Windows Phone 8 Beta program.
The Beta program that was promised but never delivered. Image from Latiff Cherono.
GDR3 is right around the corner. My theory is that AT&T will roll it out at the same time the Lumia "1520" (I say it in quotes because I don't believe that will be the number) goes on sale in November. I personally would rather see Microsoft put their effort into making sure the GDR3 release is smooth and *early* than have them try to fix what was botched months ago.
As nice as it would be to be able to blame AT&T, they are clearly not the only ones making this decision. The fault for this, IMHO, lies squarely at the feet of Microsoft. Every time Microsoft pushes out a Windows Phone release, from the very beginning, have had irritating regressions. That is mostly because of the (frankly) bull$h!t decisions around secrecy. Microsoft thinks Windows Phone 7 suffered because it was so open, but that's not the case. They suffered because it took for-freakin-ever to release WP7, and then WP8.
You would think Microsoft would have gotten their sh!t together after losing several rounds of the Smartphone wars, but sadly I don't believe that is the case. Hopefully with the Nokia purchase, they will finally fix this crap and stop shooting themselves in the extremities. Like the Black Knight in Monty Python and the Holy Grail, there are only so many limbs to cut off until you are left to die by the side of the road.