I heard that aluminum "wears out" over time. Not sure if its true. For instance if you have a loose headset that you ride like an idiot for longer than you should and it deforms your head tube - aluminum is a goner. Carbon wouldn't deform in that situation and steel probably wouldn't. Even if it did you can hammer it back in to shape with a hammer (did that with my Centurion when I dropped the frame on its head tube a year ago).

Anyway, the idea is that aluminum gets lose from the stress points of the frame in contact with other materials/ moving parts, ect... over time. I guess that means head tube, bb shell, ect... I've never had one long enough to find out personally.