I have seen a number of conversations on this site as well as other Windows Phone forums of people asking how to power off the 928 because the screen has "frozen".
I bought my 928 in May of this year shortly after it's release and not once did I ever have to do a hard power down (volume down + power button for 10-15 seconds) because of a frozen display. It is now an almost daily occurrence since the Amber firmware update. It is even worse if "Double tap to wake up" is enabled. PLEASE, if anyone at Verizon Wireless is paying attention in hear, I am begging you to report this issue with Nokia and insist on a patch to correct the problem. What was a stable phone on a stable platform is suddenly going Android on me ever day and I (along with thousands of other users) are not happy.
We have a Lumia 928, it received the Amber update, and has NOT had the screen freeze. It's been running 3-4 weeks now with the update and the only issue I have seen is the Green Screen when attempting to use the camera (one time). A reboot fixed that, and it has not happened again.
I do not have the Double Tap to wake up enabled; what are you doing when it freezes? Or does it freeze while the screen is off and you can't "swipe" the screen to wake it up? Do you have any type of password set to unlock the screen? Curious as to why it happens on some but not all devices.
The only screen issues I have ever had on my Lumia were pre-update. I don't use the thing that often, but the occasional screen flicker that it used to have has not happened since the update. And, I have never had a freeze up.
All I can say really is congratulations, and I hope both you and demmo86rt haven't both jinxed yourselves.
There is no common pattern but the screen is always unlocked when it happens. Sometimes I am talking on the phone, sometimes switching between apps, sometimes trying to post a comment to a feed in the people hub. The marked uptick of new threads in various forums asking how to "unfreeze" the 928 (how to kill power) since the Amber update rolled out can't be a coincidence. It's not EVERY day, but pretty much every day I use the device heavily I can count on a blowup.