I'm riding my first century in a week. The Reston century. I ride a Cannandale CAAD 8 with the standard road racing seat (aka "ass hatchet"). I am fine on the seat for my 30 mile group rides, but I have not riden further than that. I have a Brooks B-17 that I broke-in on my mountain bike that I converted into a commuter. With a week before the century, should I stay with the narrow seat or switch out to the Brook B-17? I can get in a 2-3 more moderate rides before the century. what do you think.

Stay with the racing seat.
Stand up pedal in a high gear when your butt tells you to.
I did a 100 today.
My butt is 66 y/o.
http://i256.photobucket.com/albums/hh187/10wheels/LeftSide.jpg

Take the original saddle off the bike, toss it in the trash and replace it with the B-17. Order another B-17 for the commuter and start breaking it in.

You've never ridden further than 30 miles, and you're doing a century?

Sorry to say that the saddle may turn out to be the least of your problems. ;) You'll probably do OK on the century itself, but will be suffering afterwards. Stock up on ice, Tiger Balm and Tylenol for the days after the ride.