The folks at Collider got a great bit of info while interviewing Stanley Tucci for Jack The Giant Slayer. Tucci says that Hayley Atwell will be making a cameo flashback appearance as Peggy Carter in Captain America: The Winter Soldier.
It’s been strongly hinted that there would be flashbacks in Winter Soldier, and Steve asked Tucci if he had been asked back to appear in those scenes. Tucci replied,
'No, they haven’t, unfortunately. I’m really sad. I know that Hayley is going back to do a flashback scene, but I have not been asked.'
Captain America: The Winter Soldier opens in 3D on April 4, 2014.