Just as a follow-up to this. It looks like the bolt holding the pulley onto the alternator backed out and fell off some how. The bolt was missing. There is signs of the pulley wobbling and hitting the alternator housing (forgot to take pictures), but the threads were still good.
I would guess that as it backed out and the pulley had some play, it wobbled a bit before everything fell off completely.
I can tell you this, it was a chore finding the right alternator in the first place. I have dual alternators, this was the one on top. Once we had the alternator, getting the old one out was... er... interesting, but not too hard. You have to take the CAC pipe off and move it out of the way. We also took the battery out (the box makes a nice tool/bolt holder too). You can get to the alternator bolts fairly easily, although the bottom, rear one is best done with a socket extension.
It was a two-person job (for us at least) to rerun the serpentine belt. I was under the truck getting to the tensioner with a 3/8 socket while my buddy was up top putting the belt into place. It's tight trying to do anything in there. I can see where people would want to take the fan shroud off, but we didn't do that... it looked sort of difficult.
So, it wasn't the worst job in the world, but it's not as easy as the 6.0 stuff I've seen... and the 7.3 looks like it could almost change itself it's so easy.
@sinner6.0L: Thanks, I didn't drive it at all except to move it to the center of the driveway to work on it. I could just see that serp belt getting mangled around something while the engine was running. Ugh.