I thought Scarlett Johansson was good at playing sexy and devious in Match Point, but I don't know if she's good at playing secretly smart, which I think this role requires. In Match Point, she was devious but ultimately kind of dumb, which worked for the blank slate nature of the part. Black Widow's gotta have some depth.
It's criminal I know, but I haven't seen enough of Scarlett Johansson to know. I liked her in The Prestige. But then, I loved everything about The Prestige.