The latest report from USA Today indicates that Hugo Weaving will voice Megatron. I can picture it now.
"Welcome to Cybertron...Misterrr Prrriiime"
They aren't telling what Megatron turns into. Previous rumours indicated he turned into this really horrid-looking "alien jet". For a robot in a disguise that wants to conceal its alien origin, that's a pretty lousy disguise. I hope he turns into a tank or something. That way they can preserve the bad-ass cannon on his arm.