A tweet by Battlefield Bad Company 2 lead designer David Goldfarb that's retweeted by the BFBC2 feed says that even if you stand on your head, they are not adding the ability to go prone in the military shooter sequel: "Fyi we will not add prone based on anything. You can stop asking now. :)"
The game players should just be spheres. And instead of a map, make it a maze. Reduce it down to be very minimal. You can't lean or go prone in a sphere. The game wouldn't be balanced for it.