It's pretty much all actors at this stage. Synthetic voices play a role during the early development of some games. Some companies will use them as “place holders” while they're doing the programming so they can see how things work together without the expense of bringing in the actors. As far as the final product is concerned, I would say 99.9% of the time it is real actors. If you hear something that sounds synthesized, usually it is made with a human voice first and has then gone through some sort of processing.
On November 6th, 2012. See this statement in context.