Yeah, I have to reiterate my belief (which I did not always have) that frame material doesn't matter. It's all in how the bike is designed.

An excellent example these days, as Horatio mentioned, is the Roubaix vs the Secteur. On the 2011 models the two bikes use the same geometry, same wheels, same type of seatpost, same elastomer inserts, both have a CF fork. They even weigh almost the same amount. The ride difference between the two is very small; I would not be surprised if the ride is nearly identical if you drop the tire pressure by, say, 10 psi on the Secteur. Oh, and the Roubaix costs about $650 more.

By the way, some people do tend to blast through centuries at high speeds, and some of them likely use racing bikes (aluminum or otherwise). I suspect it's a lot easier to tolerate a harsh bike for 4 hours than for 8....