Scarlett Johansson apparently enjoyed her time playing Black Widow in Iron Man 2, even if her skin-tight, black leather costume initially made her nervous. Johansson told Ireland Online that she's ready to play the the Russian spy again:
I hope I get to come back and do it again. The character certainly comes back. We see that she does in the comics and certainly she is a big part of The Avengers. I don't know if that is where it's going to go but she is definitely a big part of The Avengers.
Marvel's President of Production Kevin Feige has already said that Black Widow was "for sure" going to appear in The Avengers, but perhaps a solo Black Widow as well:
She has many different incarnations and different storylines so I'm hoping she will be able to continue in that way. I'm hoping that if the fans like the character and support the character, we'll see her again.