Alice Eve is the front-runner for this role
E! Online is reporting that Alice Eve (She's Out of My League) is the current front-runner to play Peggy Carter in The First Avenger: Captain America.

Apparently, Eve and Hayley Atwell (Cassandra's Dream) "are screen-testing for the second time for the coveted role tomorrow in London." The story goes on to report that Keira Knightly is no longer in the running.

Not much more is known about this casting rumor at this time.