Here's how I'm looking at this. Someone much bigger than me (which seems like the most probable way to begin this) starts running at me, in a gymnasium, let's say. I try to get out of the way, but he's too fast and too big, and all I can do is back peddle. He ends up smashing me into the wall.
I'd rule that the wall "hits" me as a bludgeoning weapon dealing 1d6 damage, plus the Strength modifier of the attacker. If the attacker had some feat that allowed for extra damage from a charge, I'd probably give it to him. Actually, I might let the attacker roll for a hit as he smashes me into the wall (on the grounds that the way that he smashes me could matter). If he rolls a 20, I'd let him try to confirm it, because, after all, I could hit my head or neck into that wall, which would deal more damage.
Dave