As of May 2013, currently nothing. At the moment it is purely a decorative item.
However, Mojang have announced that they hope to add more dragon NPCs into the game, so you never know what might happen in the future.
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The ender dragon drops it when defeated
kill the ender dragon
TONS of xp, a dragon egg, and a trip home.....
you cant yet the egg is just for decoration right now
There's no spawn egg for enderdragon.