MichaelBay.com has posted some production information on Transformers 3. The project is shooting in the states and overseas:

Spoke with Michael today and got some little info on TF3: there will be big action sequences shot in Chicago and Moscow this time around.
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Michael Bay and Steven Spielberg are working on the plot for the next chapter in the Transformers saga. It comes to theaters July 1, 2011.

Returning cast members are Shia LaBeouf, Megan Fox and Tyrese Gibson.