Producer Lorenzo di Bonaventura has been talking about the movie Transformers 3. From what he says, looks like the studios do intend to release the film in 2011 even if the timing isn't ideal for Director Michael Bay. Di Bonaventura can't imagine Transformers without Bay so I guess the studios will have to put a lot of cash on the table to change Bays' mind.
When asked about the possibility of introducing Unicron in the movie Transformers 3, he answered to IGN:
Producer Lorenzo di Bonaventura has been also praising the Mini-Crons stating that they're really cool.
So the giant Unicron (who is of the size of planet) most likely won't show up in the movie Transformers 3. But we may have the Mini-Cons!