Her name has been the most rumored for the role for quite a while now, but Samuel L. Jackson finally confirmed it.
"I don't think we begin shooting before March of next year," Jackson said of "The Avengers: Age of Ultron," the next film to begin shooting in the Marvel universe. "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 [who will play the Scarlet Witch], but I don't know what she's doing, if she's on the inside or the outside. I haven't seen a script."
Now, granted, that's a lot of brackets. but considering Scarlet Witch was the role she was in talks for, it would be a pretty huge swerve if she were playing anything but, and Joss Whedon is not JJ Abrams - his only "flare" is for storytelling.
I know they've said she won't have the costume - but based on my quick Photoshop above, I'd call that a shame.