Over at Film School Rejects, they're saying the above image is evidence of a Captain America cameo on the Incredible Hulk DVD. Director Louis Leterrier has previously mentioned there would be a glimpse of the patriotic hero, saying:
It’s an Easter egg. It’s not like oh, it’s Captain America and it changes everything. It’s still a Hulk movie, but it’s really Captain America and it’s there, you’ll see. It’s the real deal. You have to look for it.
So this probably is legitimate, even if it does just look like someone buried a tie-dye shirt. But in case you're still doubtful, or even just confused at what you're looking at, I've added some clarifying shots under the cut that should make it all crystal clear.