I'm still putting this down as rumor, but it does make sense: Samuel L. Jackson is on the set of Iron Man this week and the word is that he's playing Nick Fury, Director of S.H.I.E.L.D., the uber-spy-military organization that handles super-villain-type threats.
As you can see in the above image, the Marvel Comics "Ultimates" version of Nick Fury was without a shadow of a doubt modeled after Sam Jackson. So it makes perfect sense that Jon Favreau would bring Jackson in to play Fury in the Iron Man movie, especially in view of how involved Tony Stark has always been with S.H.I.E.L.D.
If this is true, it could be taken as a sign that the studio wants to seriously open the door to an Avengers movie, and even better, the possibility that they want to connect the individual superhero films by using the same actors.
Favreau and crew continue to make one amazing decision after another regarding Iron Man... I think this movie may end up being a far bigger hit than anyone is anticipating at this point considering that outside of comic book fans Iron Man is not a very well known character.