Downey, Jr.'s contract runs out with Iron Man 3 and he hasn't yet been signed for The Avengers 2, but I guarantee he'll be part of it, even if Disney needs to pay him with a gold-plated F-16. He's the biggest name of the cast and he was the center of the marketing efforts for The Avengers.
I've heard that Downey Jr's contract has an option for Avengers 2
, but nevertheless, he will be signed. He is the most recognizable face and the centerpiece of what is currently the most successful movie franchise currently in production.
The "phase 2" solo movies are all expected to be apart of " the avengers 2"
Iron man 3
Thor 2: the dark world
Captain America: the winter soldier
Also-- Guardians of the Galaxy