Okay, Hugo Weaving as Megatron, seems par for the course.
Peter Cullen as Optimus Prime... who's he?
So, I checked out his page, and it's obvious he's worked almost exclusively in cartoon voice work, most recently he's been the voice of Eeyore.
Fair enough, they're going for a decent work-horse voice actor... holy crap, he was the original Optimus Prime!
Anyway, watch the preview, this movie looks awesome.