If you look at the past two Marvel tent-pole movies for The Avengers they each had some sort of integration with three of the four main heroes that make the avengers.
In 'Iron-man' Captain America's shield was seen in Tony's work shop.
In 'The Incredible Hulk' there was the Captain America "appearance" and the Tony Stark appearance
Now we all know by now that Jon Favreau said on twitter that Iron-man 2 takes place before Hulk. So I would sat that would rule out any type of Hulk mention since the events haven't happened yet.
And I don't think having another Captain America reference should take place. Since it already been done twice. And not having mentioned THOR in any way in either movie only leads only leads me to think that one will happen in the next Iron-man. They have yet to make reference to him and would be pretty dumb not to since he is a founding member of the team. And would be a free easy promotion to the none Fan-boys out there to let the public know that THOR has something to do with what they are planning and its not just another super hero movie.
I would also assume that in the THOR movie thats when they will make refrence to HULK once again.