I went to see the film today and really enjoyed it.
I went to see it at the IMAX in Cineworld, Birmingham.
Now I know that it's a digital IMAX cinema, but I went to see Oblivion at the same screen the movie filled the entire screen for all but one scene.
With iron man though there were black borders at the top and bottom of the screen throughout.
Both have the same aspect ratio listed.
Oblivion was "specially formatted for IMAX" according to the Oblivion IMAX poster i got from Cineworld when i saw the film on opening day. I believe that means the frame was opened up so that IMAX viewers are getting more of the picture (it's all redundant extra space since the film was filmed with 2:35:1 in mind, but it still looks nicer on the IMAX screen), the same thing was done with the IMAX version of Skyfall.
Most IMAX conversions retain the films original aspect ratio though, hence why Iron Man 3 had black borders.
This will explain it better than i can: