Changes are made to the color of the side-view mirrors and design of the bumper it seems.
From another source, a trailer carrying Ratchet and Optimus Prime was spotted, and this was 2 weeks ago. Ratchet seems to be in green this time.
The venues from where the images were taken were not disclosed.
If you see carefully, it's Optimus Prime carrying Optimus Prime in the last image. XD
"Transformers 3" will be released next year, according to IMDB.
Images are from Megan-Fox Dotnet and Staci424's TweetPhoto.