I hope its true but so far the "proof" has just been "I think I saw on ET that the sequel has been greenlit!!!". No indication of who said it to who in the program. It could have just been speculation. Hopefully the Michael Bay blog or one of the producers will pop up online and confirm or deny the news. Even better would be YouTube footage.
The box office sales will ultimately make the final decision but with the so-so reviews of Die Hard 4 and Harry Potter until a week later, Transformers is about to have 10 or so days of box office ownage. With previous confirmations of the cast locked for sequels and scripts being discussed, a sequel is pretty much a foregone conclusion.