Galactic-Voyage spotted an exchange on the forums of Christopher Lee’s official site that confirms the actor is providing the voice of Dooku for the movie.
The site’s administrator, Juan Aneiros, had this to say:
Mr. Lee has recorded the voice of Count Dooku for this movie a couple of months ago.
It was however, the first time he had to lip synch to a character because they had produced the animation before recording his voice.
(via)