I think it'd be neat if she showed up in Captain America 2, connecting her with Bucky and the Russians who revived him and what not. That'd be neat.
But yeah, I think she should get a movie centered around her, but not necessarily her own movie. A little spy action thriller with Hawkeye would be neat, either spinning off from Avengers or set before Iron Man 2, even.
Or even a more SHIELD focused movie, with a mroe in depth look at Widow, Hawkeye, Maria Hill, Fury, and possibly Coulson, depending on the time frame of the movie.
I just have trouble picturing Black Widow leading her own movie, but if someone can convince me otherwise, I am totally open to it.