Well. I guess it depends on the dynamics behind Superman's flight.
The one fanboy science explanation I've seen suggests that Superman flies by manipulating gravitons with his mind. Down is essentialy were he want's it to be. And like Dandello suggested when he is unconscious this process stops working and his body will be subject to the gravity well and all the usual natural laws. (except that he is indestructable of course)
How that interact with atmospheric friction I don't know, perhaps he can dispel or bend it as well. Considerng the speeds he can achieve there almost must be something else
