Okay so maybe not the best screenshot you could have given us Notch. So from what i can tell these guys look like they have very long legs. Correct me if i am wrong but one seems to be holding a block? What could this mean?
Looks like this massive adventure update will bring a nice few surprises.
MMGaming.net 
They are to be called “endermen” after the “slender man” internet meme from the forums of “something awful” in which users created paranormal entity fake photos.
The slender man was meant to abduct and psychologically traumatize people. The new mob therefore is likely to be hostile at least at night if not all the time.
Addressing what is seen in the screenshot: yes indeed they are holding blocks and may well be able to place them… This may have something to do with the new stronghold feature to be implemented in the adventure update which are effectively ruins to be found throughout the minecraft world and will be made of mossy cobblestone, cracked cobblestone and iron bars – (both new blocks).
Furthermore in the foreground of the picture a “giant mushroom” can be seen – these are yet another feature to be added in the adventure update, and will be found in a new biome (the mushroom biome).
Other new features shall unclude: new farming plants; new comat system; some more water mobs; sprinting; the spike block; new food types; and most exciting of all -possible npc villages either populated by pigmen or human npc.