When you talk about century rides, I think of the fully supported charity rides and Cascade Bicycle Club events. For supported rides like that, I take my fast bike- my Litespeed Ultimate with a light set of wheels. Lately, I've been running the Campy Eurus wheels and the bike feels pretty speedy.
For a double century or longer ride, I take my Litespeed Tuscany, which has a triple. I carry a few more "what-if" supplies and extra clothes, a handlebar bag, a trunk bag, and a bento box (not to mention lights). I have Mavic Open Pros and a Schmidt hub and that weighs me down a little. I wouldn't take all that cr*p on the STP though because there's so many rest stops and so much support.
Titanium rules! I love my Litespeed bikes