With word from Anthony Williams that the US Army has issued a requirement for a PDW, it's a good time to revisit two excellent articles on the Personal Defense Weapon (PDW) by David Crane (of Defense Review), published in Military.com
Of particular interest is a white paper on the PDW presented by Walter Balsavage of Marshall Arms, Inc. [which has developed a PDW with a P90-like horizontal top magazine], which logically considers things like size, weight, and ammo. He proposes specifications for what a PDW is, and proposes changing the name to "Assault Pistol" in recognition of its potential role as an offensive weapon in asymmetric warfare.
Until we can find a copy of the actual requirement, we can only speculate on what exactly is being sought, and how many. It might be a small quantity requirement for a very narrow range of users. Since the M4 is already small enough for carrying in vehicles (by troops not needing to use their hands otherwise), a reasonable assumption is something even smaller (and lighter) is sought -- very possibly something which can be holstered (albeit in a "Mare's leg" size holster, perhaps), thus freeing up the user's hands for other duties (eg. driving, flying, laoding guns, etc.) That would narrow down the options considerably.