It appears the title of Transformers 3 has finally been revealed thanks to several books;
Transformers: The Dark of the Moon: Autobots Betrayed!
Transformers: The Dark of the Moon: Secret of the Autobots
Transformers: The Dark of the Moon: Invasion of the Decepticons
Transformers 3 The Dark of the Moon, will be released on July 1st 2011. Although the name isnt confirmed yet and the title is rather ambiguous it could be true. It is likely this is fortelling the arrival of Unicron who is the only moon sized, or related, charecter in the series.
“I'm certain I can speak for all those pledged to the Decepticon cause when I say Unicron spells one thing, and that's trouble with a capital 'running and screaming'.”
—Barricade, #6 (Vol.2)
Fun fact: The Dark of the Moon is traditionally the last three days of the Lunar cycle, immediately preceding the New Moon, and the time when the night sky is notably absent the presence of the Moon.