Apparently John Malcovich has just been signed up to star in the film. What part he'll play hasn't yet been announced.
Cast list so far is
Cast (in alphabetical order)
Warwick Davis .... Marvin the Paranoid Android
Mos Def .... Ford Prefect
Zooey Deschanel .... Tricia McMillan ("Trillian")
Martin Freeman .... Arthur Dent
Bill Nighy .... Slartibartfast
Sam Rockwell .... Zaphod Beeblebrox
I'm really not sure about having Mos Def as Ford Prefect though.