Variety has an article with the headline, "Robert Downey Jr. to Join Captain America 3," but the story inside backpedals a bit to say he is only "on the verge of signing" up for the movie, which makes sense, given how they've had to stop backing up moving trucks full of cash to get him on board, and have just started using small mobile homes instead. But what's maybe even more significant about Downey's attachment to Cap 3 is what Variety believes it reveals about the plot. So if you don't want to be spoiled about a central plot element of a movie that won't be out until 2016 (hey, there might be some people), then don't keep reading...
The article states that Downey's appearance in Captain America 3 (if it does indeed happen, since the article also discusses the rocky path that the negotiations have taken so far) will be for the purposes of setting off the big-screen adaptation of the "Civil War" storyline from the comics. I don't know that I'd be a big fan of that, because my opinion of Civil War in brief is that was too much of an overt political statement.
However, there's no reason to assume they'll go the same direction with it in the movies, so the political allegory could be much more toned down, and we'd be still be getting a lot of hero vs. hero fights which could be pretty cool (for a little while, at least). Meanwhile, as the article points out, it would make Tony Stark basically a villain in that particular movie, which could open up some very unique and cool new elements to Downey's performance. At the very least, having two of Marvel's top characters in a single movie (that isn't the Avengers) should be a lot of fun.
EDITED TO ADD: Here's the pro version of the story, which you've probably already seen on io9's main page. I can at least comfort myself with the knowledge that I started writing my version before they posted theirs.