Considering that by the end of Captain America: The First Avenger our hero was now in the present day - and that the entirety of The Avengers was also set in contemporary times - there wasn't much of a chance that Cap's belle from the shield slinger's original outing would return for the sequel, but now the actress that played her, Hayley Atwell, has confirmed she definitely won't be back.
Speaking to Time Out about her new TV project, Restless, she made clear that she isn't cropping up for another slice of the Star-Spangled Man With The Plan. No direct quotes, mind, but the copy of the Time Out article reads pretty definitively:
"She won’t, she confirms, be back for the latter’s sequel [Captain America: The Winter Soldier], which moves the action into the present day."
Considering the deleted scene from The Avengers which reveals more of Cap's troubles in the modern era, there was a tiny chance we might glimpse her in the sequel. As you can see below (and the screengrab above), it shows that at the time of The Avengers, Peggy is alive and well, but retired and living in WInchester - even though her phone number has a London code. (By the by, don't bother calling it - it doesn't work, alas...)
With that in mind, there's still a chance that audiences may see a snapshot of her instead of an actual cameo - think Natalie Portman's Jane in The Avengers - but that's all conjecture. Hell, perhaps there'll be an older version of the character with a new actress, or Hayley's pulling the wool over our eyes, Guy-Pearce-in-Prometheus-style, and will rock up in heavy make-up... Then again, perhaps not.
Captain America: The Winter Soldier is set for an April 4, 2014 release.