Interviewed by The Advertiser, an Australian newspaper, Weaving says he's already recorded the voice for Megatron in the new movie, and adds that it's a huge secret. Which he's now given away. Asked how Megatron can be back after dying in the first movie, Weaving says:

I think in the last one, doesn't Optimus Prime pick up a little bit of, a shard of the cube, or the thing that's left, and he takes it with him. Maybe there's something there… but I don't know. [Laughs]

After Weaving came clean, director Michael Bay posted on his site's forum and admitted that, yes, Megatron really is back:

Okay cat is out of the bag. Megatron is back —-- but you will only get to see him from the long lost past. We go way back in time as this movie explains the mythology of the Primes. He is not a tank like everyone suggests, but an alien vehicle. But sadly he does not has much screen time.

[via Seibertron and SciFi Cool]