At an special screening of Iron Man and a live commentary by director Jon Favreau and Downey Jr., covered by FilmCrunch, a number of sequel and Avengers hints were pointed out:

Throughout the movie, the actor-turned-director pointed out and confirmed various scenes which hinted at both The Avengers movie (visible S.H.I.E.L.D. logos) and the sequel (he noted that the terrorist cell in the movie is merely one branch of Mandarin’s group).

Downey served to be more entertaining during the commentary than his character in Iron Man, which I didn’t think was possible.

With the DVD release and the various on disc commentaries, we can look forward to more details about the forthcoming sequel.