Anyone who thinks that the Iron Man helmet is inaccurate is forgetting one thing:
You haven't seen the movie yet!
This is an Avengers helmet, not an Iron Man I helmet or an Iron Man 2 one. Maybe, just maybe he gets a new helmet. And maybe those spy shots we have from the set, early on, where people were carrying two slightly different Iron Man helmets around (One that looks suspiciously like this one) could indicate something like that might happen.
And MAYBE if you took the time to compare the helmet to the head of the Mark 7 statuette we've seen, you'd notice it matches...
And MAYBE the filmmakers want the suit change to be a bit of a surprise so they rendered the CGI for the posters and trailers using the old helmet model...like they did for the original Spider-Man trailer, keeping the surprise of the "Human Spider" costume a surprise for the theater audience, but rendering some of that footage using the full suit just 'cause.
Keep up the great work eFX. Looks like I'll have to save my pennies.
-Nick