While a Super Meat Boy / Giana Sisters crossover has yet to be officially announced, the rating board has a fair amount of information attached to its rating; the listing claims that Giana Sisters vs. Super Meat Boy will be a PC title, published by Headup Games. We've provided a screenshot above, but you can see the listing for yourself by checking out the board's website and searching "Giana Sisters vs. Super Meat Boy."
Super Meat Boy and Giana Sisters: Twisted Dreams are both independently-developed 2D platformers, so a team-up isn't exactly out of the question. Still, it's an ... unexpected team-up to say the least.
Update: Rather than an entirely new game, as reader "spunior" pointed out, it could also just be a bundle of both franchises under a new SKU, hence the new rating. Amazon.de lists the product as "Clash of Games: Giana Sisters vs. Super Meat Boy," with a release date of June 26 - so either way, we probably won't have to wait long before things become official.
Update 2: Yup, it's a collection. A press release from Headup Games confirms that Giana Sisters vs. Super Meat Boy is not, in fact, a new game, but a bundle of Giana Sisters: Twisted Dreams and Super Meat Boy. It will be available in Germany on June 26.