Dedicated Diablo 3 players have already discovered the secret level that we all knew would be hidden away somewhere in the game. The Diablo series has a history of such things, with rumors of a secret cow level than never panned out in the first game leading to the inclusion of an actual secret cow level in the second game. Moo Moo Farm is no more in Diablo 3, however. It's been replaced by Whimsyshire, a colorful land filled with deadly unicorns, stuffed teddy bears, and flowers.
Getting to the hidden area -- and earning the achievements and loot that it offers -- involves finding an assortment of random object spread out between acts 1-4 of the game, and then using those objects to somehow open a portal. You can find a rough set of instructions in the Battle.net forums (via Polygon). You can also see what this ridiculous location looks like in the video up top.