A friend of mine down in the Presidential Towers snapped this pic today. Although this is completely a hunch, I am certain enough to claim that this is most likely the base of operations for the Transformers 3 production unit.
This gaggle of buses and trailers showed up overnight last night, Wednesday June 30. It is definitely a movie production, and considering the scope of this setup, it most definitely has to be Transformers.
Those trucks are probably not for Transformers, Cheaters with Vince Vaughn is filming at Sepia right down the street from that lot.