Thanks to Carnivac (and also supported by TFW2005 thread), much of the voice cast for the various Transformers has been verified. Many of the robots in the movie don't have lines and it turns out some of the information we had was incorrect (not really surprised considering how little attention the voice cast gets).
I wish Paramount would release the credit list (like they did for Star Trek) so have something definitive to link to to clear up any probable confusion on who did what in the movie.
Likely voice cast: (but some not officially confirmed) (clicking their names brings you to their IMDB.com profile)
Arcee - Grey DeLisle (not Susan Blu)
The Fallen - Tony Todd
Jetfire - Mark Ryan (not John Turturro)
Mudflap - Reno Wilson
Skids & Wheelie - Tom Kenny
Soundwave & Devastator - Frank Welker
Sideswipe - Andre Sogliuzzo (not Jason Griffith)
The Doctor (Scalpel) - John DiCrosta
an ancient Prime - Kevin Michael Richardson
Demolisher - remains unknown
Its worth hitting up the profiles for these actors because of the long careers many of these actors have had. Even if you don't know the name, chances are you will recognize the face or their work. As far as Transformers connections, Kevin Michael Richardson voiced Omega Supreme in Transformers: Animated, Reno Wilson voiced Frenzy in the first movie; Mark Ryan also voices Bumblebee in the movies and Ironhide in the video game. Tom Kenny (other than being SpongeBob SquarePants) also voice Isaac Sumdac, Starscream, various Seekers, Scrapper and more for Transformers: Animated. Of course Frank Welker (G1's Megatron, Soundwave, Galavatron, Rumble and more) doesn't really need an introduction to Transfans but his storied career is massive and covers work you would have never had thought had anything to do with him.