I'm having the same issue as all of you. I updated to 5.1 a couple of weeks ago without issue, but the last three days has been nothing but constant resets and reboots, and non-stop "Optimzing Apps". Sometimes I put the phone down on the desk in front of me with witnesses and we will watch the phone reboot itself over and over and over without anyone touching it. It will try and boot, start doing its thing, fail, turn off... 15 seconds later it will try and boot back up again, repeat repeat repeat. Sometimes it will stay off for over an hour before it tries turning itself back on again, randomly. It works maybe one out of every 60-80 times it tries, and I'd say in the last 24 hours without exaggerating it's probably done this about 200 times or more.