It sure sounds like Elizabeth Olsen, the rumored Scarlet Witch for Avengers: Age of Ultron, is indeed going to be playing the role. First came an interview with Samuel L. Jackson, Nick Fury in the Marvel Cinematic Universe, where he simply told the Wall Street Journal, "I know we're shooting in London, that James Spader is Ultron and going to be the bad guy, and that we added Ms. [Elizabeth] Olsen, but I don't know what she's doing, if she's on the inside or the outside. I haven't seen a script."
That certainly implies that Olsen's deal is all-but-done to play Wanda Maximoff, the Scarlet Witch. As to his "inside or outside" comment, that appears to be referring to whether she's a villain or a hero. The character has played both roles in her fifty-year history in the Marvel Universe, multiple times. It's quite possible that we'll see her and her twin brother, Pietro aka Quicksilver, do the classic comic book switcheroo during the film.
Other outlets have since confirmed that Marvel Studios and Olsen are "close to a deal," including Deadline who cites a source "close to the situation" saying, "her deal is “just a matter of formality before it’s done.”
Marvel Studios has not commented, however.