I've moved this to a thread of its own in hopes of attracting a few more comments.
Basically, the number of states that offer reciprocity on driving licenses with France has grown from 6 to 10 over the last 15 years. So I wouldn't hold my breath on California joining the list anytime soon. (Though sometimes things happen unexpectedly.)
If you have an exchangeable license, you go in to the prefecture to do so. Each prefecture has its own routines and procedures. Where I live, they only do license exchanges in the mornings on certain days of the week. (Like Peter said, welcome to French bureaucracy.)
To be honest about it, it isn't a bad idea to hit an auto école and take a few lessons before you go for your written and driving exam. There are a number of "interesting" driving regulations in France that tend to flummox Americans, and knowing how things are supposed to work can really help. (First on that list is "priorité ŕ droite" which is considered a god-given right in most of Europe.)
And forget about getting information on any administrative procedure via e-mail (even in French). Most fonctionnaires don't respond to e-mail inquiries. The French government has some excellent websites on a variety of topics. Start with
SERVICE PUBLIC - Particuliers and chances are you can find links to most of what you need to know - but only in French. Get used to simply going in to make queries or get information. In person visits generally yield the best results once you get to France.
Cheers,
Bev