well, acording to Seibertron.com, it's down to Frank Welker and Hugo Weaving at the moment...
Who's Hugo Weaving?
indeed there's a few scenes that are ripped straight out of the original series in this
Anywho, I'd kinda want David Kaye to play Megatron in this movie, I've always liked him better than Welker. Same for Prime, I wanted Gary Chalk to get his voice, but that's probably because I grew up with him as the voice of Prime.