Under the Settings/Power/Power On setting it lists the various inputs. If you check the one that you want to have start automatically when the TV is turned then that input should appear when you turn the TV on and it completes its start up sequence. This function is not working.
It works fine here on the Roku TV listed in my signature.
I guess the first thing I would try would be to change the setting to a different input, restart the tv to see if that "took", then reset back to my desired input and restart again.
To restart: From the home screen.. Settings.. System.. Power .. Restart system.