The Binding of Isaac's first and only expansion, "The Wrath of the Lamb," at some point in "the next 2 - 4 months." At least that's according to Mr. McMillen himself, speaking with Joystiq in an email exchange last evening. "If we land the 3DS port, it will include the expansion for sure," McMillen added.
As for the planned Nintendo 3DS version, McMillen doesn't have much in the way of solid news just yet, but the project is moving along. "The only news I have on the 3DS is that it seems possible but it's still going through higher ups to make sure there won't be an issue with the 'religious' aspect," he said. And if the 3DS version doesn't work out, there are apparently other plans in the books.
"If we don't land the 3DS, the publisher I'm talking to has expressed interest in other digital platforms and is currently contacting them as well about the port." Considering the game is already available on PC, Mac, and Linux, we're presuming that means ... somewhere else (XBLA, PSN, Vita, and mobiles come to mind).
He also offered some details on the upcoming expansion, including the news that it will contain "about 50 percent more content." McMillen said the content comprises, "enemies, alt chapters, bosses, items, characters, challenges, new happenings and mechanics as well as a ton of other little details," and after that "nothing else" can be done with the game, "so I'll close the book on it." Which is rather fitting, considering the game's thematic inspiration, no?