Although Sony seems to be telling owners that the problem is defective hardware, it doesn't seem true. My reboots, and everyone else who has posted about them, report they started happening right after the firnware update (2 updates ago). Seems very unlikely (although not impossible) that all of our set went bad at the same time as they all probably have different hours of use, different manufacturing dates, etc.
I'm still hoping for a firmware fix but as of now, my set occasionally reboots usually about 5 minutes after power on. If not fixed after another firmware update, I guess I'll have to break down and call Sony.