I don't have much experience with Al but I'll take a stab at this anyway. Cannondale 'metal finishes' their welds. All this means is they grind them down to blend in with the base metal(tubing or dropout or whatever).
Here's a short tutorial (look for posts by SELLERS) by a relatively young and unknown master of this process. Bear with his poor grammar and lack of punctuation; it's worth it if you really want to learn the process. The thing about Al is it tends to 'load up' on your abrasives. Basically Al chips clog the sanding wheel and it loses it's effectiveness. Probably not an issue when your using little disposable sanding discs. I'm gonna try to find a chunk of Al to take to work tomorrow just to see how it reacts to grinding. The main thing to remember is not to grind into the base material at all. Only the weld.