Actually, since Zaphod has two heads, maybe Statler and Waldorf should be playing him instead.
Of course Hollywood wants someone like Jim Carrey to be in the film, because they're all about selling films based on having big stars. While I like Carrey's outrageous humor in the right film ("Liar Liar," "Bruce Almighty"), his style would be all wrong for this project.
I don't know who should be Arthur, but a couple of Muppet Show guests come to mind for other parts. I think Steve Martin would make a good Ford. Yeah, I know, he's supposed to be British (or rather an alien pretending to be British), but for some reason Steve's dry sense of humor seems like it would really work here. Also, I think the movie may need a narrator and a voice for the Guide (both played by the same person), as so much of what's funny in the book is author Douglas Adams' humorous asides and quotes from the text of the Guide. For this role, I think John Cleese would be perfect.