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.