Version 1.4.2/12w37a (2012): Added as a hidden and disabled feature, human heads can now have the skin of a specific player. A line in the language file implies that there either used to be, or in a future update, the possibility of obtaining a specific player's head. Player heads do not/will not drop in vanilla, but mapmakers/modders/etc. can make them by adding an NBT string tag "SkullOwner" -> (player name) on the skull items.
November 7, 2012: Jeb states that the official opinion is that "[dropping] player heads [on player kills] are a bit too gory for Minecraft, so it's better suited for a plugin or mod."