I found this interview with Peter Cullen. It's a pretty run of the mill Transformers: Prime interview, talking about the character of Optimus Prime and it doesn't reveal too much more that we didn't already know. It's this response to the second to last question that got my attention: Does this mean that he just went down and spoke Optimus' lines instead of Mark Ryan (who read for all the robots in the two previous films on set) or does Peter Cullen get an on screen cameo? I don't know, but I think it's cool either way. Source was the Transfromers Live Action Movie Blog.