Ford Prefect will be played by Mos Def, shown here (second from right)
Trillian will be played by Zooey Deschanel, shown here in "Almost Famous" (and really, the 70s stewardess look is almost quasi-futuristic, don't you think?)
And just in case I hadn't been enough of a dork today, the suggestion of "Ryan Stiles as Ford" on the SA Forums brought me and E. to discuss WHOSE GUIDE IS IT ANYWAY?
Starring Paul Merton as Arthur, Ryan Stiles as Ford, Chip Esten and Brad Sherwood as Zaphod (Chip acting as Brad's arms, "Helping Hands" style), Josie Lawrence as Trillian, Tony Slattery and Mike McShane as Majikthise and Vroomfondel, Colin Mochrie as Slartibartfast, Greg Proops and Sandi Toksvig as Benjy and Frankie Mouse, Steve Frost as both Deep Thought and Marvin, and Clive Anderson as the The Book saying goodnight: goodnight.
Oh, and Wayne Brady as far from this project as possible.