Just a few days ago, the Space Coast Film Commission posted that they are ready to know seek applications for various jobs that will open up when Transformers 3 production arrives. The listed film dates were September 19th to October 5th. Today the site added a change today that shortened the schedule by a week to September 29th to October 5th.
No idea if the change was because the initial post was incorrect or because of a change in the shooting schedule. I guess that means that production will remain in Detroit for at least another week. Thanks to @kimmyscraftfun for the heads-up.