“I’ve only done the opening,” voiceover legend Peter Cullen told MTV News, updating us on the lengthy process of recording lines for the film. “[Michael] Bay brought me in to get them started on some of the animation part of it. I think they require a voiceover for it, [and then] we’ll get into the main stuff more towards the end of the fall…November, perhaps.”
And while Cullen will remain off-camera, like in the first film, some of him will find a home on the synchronized face of his alter-ego, Prime. “They focus a camera on my mouth,” he exclaimed. “It’s like ‘Oh my God, they’re looking into my mouth!’ It’s like going to the dentist.”
source http://moviesblog.mtv.com/2008/09/25/optimus-prime-takes-shape-for-transformers-2-as-voice-artist-peter-cullen-returns-to-work/