Update (from Peter Sciretta): The Hollywood Reporter has word that Smulders is in "final negotiations" to take on the role, a deal which has options for a number of future Marvel movie appearances. The Avengers is scheduled to start shooting in late April 2011.
It's looking more and more likely that Nick Fury's S.H.I.E.L.D. assistant in Joss Whedon's The Avengers is going to be played by Cobie Smulders. ...