It seems that someone TFW2005 might have discovered the official title of Transformers 3 thanks to a leak via Amazon.
The title - Transformers: The Dark of the Moon
The discovery started with three children books posted on Amazon with the same suffix title. The books are called Transformers: The Dark of the Moon: Autobots Betrayed!, Transformers: The Dark of the Moon: Secret of the Autobots, and Transformers: The Dark of the Moon: Invasion of the Decepticons. Keep in mind a children's series of books were released with the second movie. The theory is further supported by the discovery that the domain transformersthedarkofthemoonmovie.com was registered about three weeks ago but the owner remains unknown.
While the title does seem a bit pulpish, it really isn't much different in style as "Revenge of the Fallen". It does fit rumors that the movie's plot ties directly into the 1960's missions to the moon as does the current filming at the Kennedy Space Center. No idea how long it will take for this to be confirmed or denied. If this title is accurate, maybe someone can talk Michael Bay and company into providing the "real" title of the first Transformers movie (like Star Wars: A New Hope). Thanks to Udana for the heads-up.
Update: Collider is reporting they were able to get official confirmation that this is the title of the movie, "We have now confirmed the title with sources close to the production." IGN believes the confirmation came from Lorenzo di Bonaventura who often provides content for the site. Thanks to Brian for the link.