If I read the first post correctly, than a lot of people are misunderstanding it. This topic isn't about whether Iron Man will play the most important part in the movie or be the main character, as one can assume that the main character will probably be Cap--instead, this topic is asking whether Iron Man will be the fan-favorite character, the one the fans/general audience/etc. is rooting for.
Now Iron Man's my favorite anyway, so I'm a little biased, heh. However, based on the popularity of the movie, Downey Jr.'s performance and the strong reactions to the cameo in The Incredible Hulk, I think it's a good bet that he could very easily be the fan-favorite, much like Batman, when the Avengers movie rolls around.
If that comparison isn't good enough, look at it this way; the general public always goes for the more cocky, rebellious character. In another comics example, see the preference people have for Wolverine over Cyclops. Not enough? Go back to the original Star Wars and see how many people preferred Han Solo to Luke.