Quote Originally Posted by TFW2005
Full voice cast:

Optimus Prime - Peter Cullen
Bumblebee - Does not speak in the film
Ratchet - Robert Foxworth
Ironhide - Jess Harnell
Sideswipe - Andre Sogliuzzo
Arcee - Grey DeLisle
Jetfire - Mark Ryan
Mudflap - Reno Wilson
Skids - Tom Kenny
Wheelie - Tom Kenny
Jolt - Does not speak in the film

Megatron - Hugo Weaving
Starscream - Charles Adler
Soundwave - Frank Welker (but lacking the vocodered voice)
The Fallen - Tony Todd
Scalpel "The Doctor" - John DiCrosta

Demolishor also speaks, although the various other Constructicons do not. Devastator reportedly just grunts and roars, so there is no confirmation if Frank Welker is providing his voice. Additionally Kevin Michael Richardson provides the voice of an ancient Transformer seen in one sequence.

Some surprises here as it was originally stated John Turtorro was Jetfire, Susan Blu was Arcee etc etc, so good to see the misinformation trail went right up to the film's release.

Kevin Michael Richardson is one of my all-time favourite voice actors (I'd shag his voice if I could), and when he spoke in the film I swear I nearly fainted. His voice suited the character beautifully.