The Sacrifice DLC has been released for L4D1 PC and L4D2 on 360 and Mac.
This means playing as the original characters from L4D1 in L4D2 like you never left the game at all! You also get the amazing chance of using melee weapons with your favorite L4D characters.
As most of you already know, Bill is seen "laying around" in The Passing DLC. I guess everyone can put 1 and 1 together to guess what The Sacrifice is really all about.