This is probably old news, but it is from an official source.  Several months ago, we had word that Bernie Mack had been cast in the TF Movie, then a site or two reported that he was not...  Don Murphy.net has this to say, with a little more elaboration on his role:

" Bernie Mac and Josh Duhamel are set to star in Michael Bay's "Transformers: The Movie" for DreamWorks and Paramount. Live-action production starts lensing this month, with release scheduled for July 4, 2007. Budgeted well north of $100 million, "Transformers" is based on the comicbook, cartoon and Hasbro toy line popular in the 1980s. Story centers on the central saga of the Autobots vs. the Decepticons.

Mac will play a salesman who sells Shia LaBeouf's character his first car, a Transformer. Jon Voight, Megan Fox, John Turturro, John Robinson and Travis Van Winkle round out the cast.

Tom DeSanto, Don Murphy and Par-based Lorenzo di Bonaventura are producing the film. Steven Spielberg and Hasbro's Brian Goldner exec producer. Alex Kurtzman and Roberto Orci penned the screenplay..."