This has not been mentioned but in the IMAX version, when Megatron comes out of the water after being repaired I think it's extended a bit, I think you see Mixmaster fly out of the water too.
the biggest difference, the screen expanding on the Devastator and Forest Fight scenes AND of course the Crystal Clear resolution during those scenes
Actually, I THINK I noticed a second scene besides the Grindor scene. There was a part during the desert battle where Sideways is spinning around shooting and I think he tells some soldiers to get back. I think the scene ends righ there. In the Imax, it pans over and you see Lennox telling Ironhide (who is in truck mode) to "go down the middle" or something and he drives off and we get a good scrolling shot of the Arcee twins with him. I think it leads up to the scene where bonecrusher shoots one of the twins.
Definitely some additions to the forest battle as well as the scene of Devastator combining, on top of the screen getting bigger for both of those parts. I noticed Prime getting picked up by Grindor, Starscream falling over in pain when Prime kicks off part of his face, and an additional second or two of Prime being ganged up on before getting launched in the air. They threw in some additional angles of Devastator combining too. I saw the movie three times in a standard theater and once in IMAX. It was very easy to follow, but previous viewings will do that.