Update 3/29: For those still interested, the last two keychains on the Key Party website were Claptrap's eye and a companion cube for GLaDOS. Now we'll just have to wait until Monday to see how much information on Poker Night 2 Telltale plans to release, and if the Key Party theme actually ties into the game somehow.
Update 3/27 #2: It turns out it is indeed Poker Night 2. The box art and screenshots have been found on the xbox.com website. So, it seems the game will be available for at least PC/Mac and Xbox 360. The characters are Sam, Brock, and Ash (as already revealed), as well as Claptrap from Borderlands 2 (rather than Handsome Jack as previously assumed). GlaDOS from Portal seems to be taking the place of Reginald Van Winslow as the host.
Update 3/27: The Key Party site now has a keychain with the Necronomicon for Ash.
Update 3/26: The website hasn't been updated yet but the key was revealed on Adult Swim, and retweeted by Telltale. As expected, it's Brock from Venture Bros. (the keychain of his big knife, which will also be a Team Fortress 2 unlock). It will be interesting to see what keychains they use for Evil Dead's Ash, Borderland's Handsome Jack, and Portal's Chell, Wheatley, or GlaDOS (that one's harder to pinpoint, since the boots can belong to any of them).
Update 3/25: The first key is up, and it's a key to a DeSoto. So, that means it's Sam's. So it looks like The Key Party is indeed the Poker Night sequel (which the background for the key text pretty much gave away anyway as it matches Poker Night's user interface). Although whether it's another card game or an entirely new genre altogether is still up in the air.Original post 3/22: Telltale has unveiled a website called The Key Party. The website claims that the answer as to what The Key Party is will be revealed on April 1st.
However, we'll most likely get some clues before hand, since if you mouse over the key, you'll see that the "guests will start to arrive March 25th, 2013".So what is it? Poker Night 2? Fables? King's Quest? Theodore Dudebrough's Excellent Adventure? Your guess is as good as ours.