Audrey Tautou did a find acting job in Dirty Pretty Things, and there was nothing cute about that movie. I've never seen her project much Sophie-like intelligence and charisma, though; the producers probably just wanted the French actress currently most popular in the United States. I think SnowflakeGirl's Julie Delpy idea was better, but at least they chose a real French girl!
I think Tom Hanks will be okay. Honestly, the characters in the book didn't have too much personality for acting to even be that important. The movie will be plot and suspense driven (just like the book) and regardless of whether it's decent or not, people will line up to see it (just like with the book).