The Binding of Isaac? You probably shouldn't be, as that's kind of the point, but there's an expansion on the way should that be the case. McMillen revealed the title of the update via Twitter, naming it "The Wrath of the Lamb," and offering roughly zero details about its content. He did note that he pulled the name from the Catholic Bible's Book of Revelations, and that "whoever wrote this part of the Bible has a profoundly epic imagination."
The only info we've got so far is that it'll cost $3 and will apparently feature a whole mess of new items (over 205), including something called "trinkets" that will passively buff your character. No specific release date is given either, but it's intended to launch at some point in 2012.