I'm going to change my last comment, I've seen it happen twice while watching a Netflix movie with the latest 6.0 (second version) update (on a ATV2).
Its not a regular reboot, its a very short, 5-10 second reboot with the front panel light blinking. When its finished, it goes back to ATV main screeen.
As I said in my previous post, I had no issues with both the 6.0 updates. Both installed flawlessly.