This is very interesting. While thanking a sender for this model of his head, Mojang's head artist may have leaked an early version of the official Minecraft LEGO set. Since the set was not acknowledged in the tweet, it may very well be something that LEGO sells already. Notice how the white space in the black LEGO build on the bottom looks identical to a Minecraft pickaxe. Mojang also has not met with LEGO to discuss possible set designs, so if this is a Minecraft LEGO set, it is probably an early prototype that may never see the bright lights of a Wal-Mart shelf.
Source: TwitPic