It appears that I was just in a lucky period, because I've had this happen to me three or four times over the last few days (after going more than a week without an occurrence after I deleted NetCounter). I've found that if I turn on airplane mode and then turn it off again, I don't reconnect -- but after a minute or so, the phone spontaneously reboots! The same thing happens if I try to power down using the top switch -- the circular arrow stops spinning after a while and a minute or so later, the phone reboots instead of powering down.
It may well have to do with wifi, as I leave mine on nearly all the time. Whatever the cause, it is very annoying, and I hope a future update will fix it.