I have had my Apple TV update automatically.
It prompted me telling me there was an update available and asked if I wanted to update now and I selected NOT to update now. But it just kept asking me telling me there was an update available and asking if I wanted to update and I kept selecting NO and all of a sudden it just started to install the update all on it's own.
And yes I'm sure I was selecting the option NOT to update now
If you want to prevent your aTV from updating automatically, just get a free patchstick here. During installation, it automatically disables automatic updates. You can re-enable updates using the remote control.