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. :)"
Hey don't get me wrong I hate the compromises made for the console and DLC greed as much as the next guy, but lack prone is one of the few things they did get right. It just wouldn't fit in this type of destructible detailed enviroment.