I noticed some age in Cullen's voice as well.
I would've liked Welker doing Megatron's voice again, but I'm not so much of a fanboy that this choice upsets me that much. I already have enough other issues if I want to go all fanboy-freaky. And this isn't G1 Megatron anyway. And though the various Megatrons have all sounded similar, I think Hugo Weaving can pull off a close enough voice, or some voice that will fit this radically different Megatron. I'd probably be more interested in what he says (like his lines towards Optimus, Starscream, etc. and personality) that how he says it.
Despite Hugo Weaving's presence in two major film trilogies recently (The Matrix, The Lord Of The Rings) as well as V for Vendetta (okay, we never see his face, but still), I'm not sure if he's an A-list star. I wonder if his name, or any name (other than Spielberg and Bay), will be used to sell the film.