As of right now, Viggo Mortensen is not in Peter Jackson's Lord of the Rings two-part film prequel The Hobbit and nor does he expect to be asked at this stage despite many of the original cast returning for cameo's/supporting roles. Morten claims he was asked a few times by the producers for a reprisal of Aragorn before filming began but since they've been shooting the movie in New Zealand, the call has never come. In an interview to promote his new movie A Dangerous Method, Mortensen tells Movies.com;
"At one point, the producers asked if I would do it and I said sure, if Aragorn is supposed to be in the bridge story, because he's not in 'The Hobbit,'" said the actor. "But I've never been asked and they're shooting the movie. I'm not in it unless there is some last-minute plan they have, but I thought I would have heard of it by now."
Previously Peter Jackson hinted that there was room to squeeze Aragorn into The Hobbit story, probably the bridge between the two parts of the film, but the actor continues;
"Orlando Bloom and Cate Blanchett shot something, but they're elves and don't change as rapidly. As you know, Aragorn is half elf and also lives a couple hundred years or more and he could be in a bridge, but I have to assume it isn't going to happen."
The Hobbit: An Unexpected Journey is set for a December 14th, 2012 release with The Hobbit: There and Back Again coming December 13th, 2013.