Notch said he wants to get Minecraft on Steam, so maybe Valve will come up with some deal once Minecraft is on Steam where you get a free hat for buying it.
Gibus Mark I = Actual Ghastly Gibus, just dominate a player with the Gibus, IT'S SO EASY, EVEN A CAVEMAN COULD DO IT
Gibus Mark II = Alien Swarm Parasite, which is common because Alien Swarm is free and you only need to complete two missions
Special hats like these need to be harder to get :(