OP here. Well mine finally dropped. I found it on the floor overnight.
With my 3GS, I typically put the iPhone on top of my leather wallet on my dresser. Out of habit, I did the same with my 4S but because the back is so slippery, it must have slid off the wallet and fell about 3 feet onto the hard wood floor from my dresser.
Amazingly there is the barest of dents on the corner of the back plastic. I'm guessing when it fell, it almost fell at a completely horizontal position with the floor because there would be a giant dent if it fell right on the corner.