This post is half solid, half rumor. The part about the Meat Boy shirts coming to Xbox 360 avatars is definitely legit. This image, spotted on Super Meat Boy's twitter, is the real deal.
The second part part of this post is probably equally legit, but due to my super-serious journalistic standards, I'll have call it a rumor for now. Though I doubt DIY Gamer's would lie in reporting of a Spelunky sighting in Super Meat Boy, until I see it myself, I can't be 100%.
Come to think of it, one of the shadowy figures from the Super Meat Boy character select screen did resemble the spelunker from Spelunky. With his panache for running, jumping, and dying, he'd be a perfect fit for the Meat Boy world, and his alleged special move, a flash bomb that ignites him a into a flash of dashing power, would make a lot of sense for Super Meat Boy's fast paced gameplay.
Only a few more weeks until we find out for ourselves.