@Seed,
As far as I know, the release to theaters that are 2D occurred at the same time, December 18th. Go to
www.fandango.com to find theaters that are not IMAX, and the 2D was not simultaneously shot but a process which fits the wider ratio screens which is a digitally merge of the left and right image required for 3D. I'd really have to to some research to find out if this is 100% true, but I did read it at one time -- however, several months ago.
The screen ratio of IMAX's huge screens is actually close to the aspect ratio 4.3 which is like a vintage movie or the standard TV screen that is not a new 16.9 wide screen HD. On the wide flat screens, I would have to think it would be Super Panavision 2.35:1 (in other words, over 2 1/3 times wider than the image is in height), although some of the larger theaters can project up to 2.65:1. Whether you're losing some image at the top and bottom or gaining image on the sides, I'm going to say that you loose the top and bottom of the IMAX image and it was shot to not have anything but peripheral image in those areas. So, more jungle, sky, outer space, interior space ship, or whatever is in the scene as a background.
4.3 is, of course, 4 units wide by 3 units in height, so about 25% wider.