A Black Widow solo movie starring Scarlett Johansson is being discussed as a possibility after Joss Whedon's Avengers movie, according to The HD Room. Marvel Studios President Kevin Feige is quoted at an appearance regarding the upcoming Iron Man 2 DVD/Blue Ray release.
"We've already started discussions with Scarlett about the idea of a solo movie and have begun putting together concepts," Feige said. "But The Avengers comes first."
This could be very good news if they put a good story together, and it could further develop the shared movie universe Marvel has been building since the first Iron Man movie. And it's so early in the game it could end up being either a prequel or a story set at any point of the Marvel movie universe--the events in Iron Man 2 appear to have taken place before Incredible Hulk, which was released first, for example.
While we're waiting for her next appearance in Avengers, here's a good place to brush up on her origins.