Terry Gilliam would be my choice.
I'll second that.
As far as movie plot, etc goes - I think for a game like Fallout the real mis-step would be in trying to make something that followed any of the existing storylines too closely. A key element of the franchise is at least a certain amount of open-endedness. No movie is going to be able to conform to a significant majority of the players' experiences in the game (not to mention that unless you've painstakingly created a Vin Diesel head to roleplay with in the game, the main actor isn't going to look very much like your own character.) These games are very personal experiences for anyone playing them. I've gone through Fallout 1 and 2 dozens of times, and yet I still have my one playthrough for each that I consider the "real" way that things worked out in my head.
The best way for them to go would be to come up with their own totally original storyline that tried to hit on some of the key themes of the universe, rather than trying to pull anything directly from the plotlines of the existing games. I mean, certainly some common elements would play in a hypothetical Fallout movie: It would probably start out with someone being sent out from their Vault for whatever purpose. At some point they'd run into Supermutants and uncover their plot and origins. And they would enlist the help of the Brotherhood of Steel in stopping the menace, and possibly a conflict between them and the Enclave, as well. That stuff I'd imagine is sort of a given for any Fallout movie.
But likely the best way to go about any of that would be to even have the movie take place in a completely different area than California or DC.