He's been working with Mark Ruffalo, but also doing mysterious other work. 

Yesterday I spent time with Andy Serkis at the Alamo Drafthouse's Dawn of the Planet of the Apes event... which unfortunately just wasn't the right place to ask him about the breaking news that he was playing some sort of role in The Avengers: Age of Ultron

We knew that Serkis was involved in The Avengers through his consulting company, The Imaginarium, which works with actors doing performance capture, and that's part of why he's on set. He's been working with Mark Ruffalo as he reprises his role of The Hulk. 

Performance capture was used on the previous “Hulk,” but not to the same extent Mark is using it now. There are tools and ways of working with an actor that makes them feel that they own the character. And a character the size of the Hulk needs ownership, which Mark has now.

But more than that, Serkis is playing a role in the movie. What role? He won't say - he won't even say if it's a performance capture part or if he'll be seen directly onscreen. There are a lot of performance capture opportunities in The Avengers, as Ultron creates an army of robots to battle the Avengers, but it's possible that Serkis just shows up in a cute cameo that means nothing. 

His work in Dawn of the Planet of the Apes is extraordinary; he and the animators at WETA have together created a new version of Caesar that is realer-than-real - he's just a character who exists and acts and is totally, completely believable.