Just in from Variety's Jeff Sneider (@TheInSneider): "FYI, Scarlett Johanson's [sic] Black Widow is CONFIRMED to appear in CAPTAIN AMERICA 2: THE WINTER SOLDIER." (!!!!!!!!!!!!!!)

Not much else to report just yet as the news is pretty fresh, but here's what Miss workerbee73 and I have found so far:
"Teresa Palmer, Imogen Poots, Alison Brie & More Testing For 'Captain America 2'; Scarlett Johansson To Possibly Appear"

"Scarlett Johansson to Appear in Captain America Sequel"

Now we just need Renner's Hawkeye and we will be SET, yeah?