Actually, as far as I can tell, how do we know that we do see things up side down? Certianly the image on the retina is inverted, but interpretation of that information is the job of the visual cortex.
Does the brain really invert it, or do we just process it as is and perceive it the correct way up since that is what we are used to?
I know some experiments were done in which people had to wear a special image inverting headset the entire time they were awake. After a few days they were entirely used to it.