same exact thing happened to me.
http://www.renntech.org/forums/topic/24050-stuck-in-an-alternator-swap/page__p__125621__hl__stateofidleness__fromsearch__ 1#entry125621
I thought I had posted on here for help when it happened, but I think it might have been before my 986forum days.
my Indy ended up fixing it for me, but it appears I didn't seat the bottom right bolt in the "channel" properly. It allowed it to "shift" and swivel just enough to let the belt come off a bit and start grinding into the plastic cable cover just above it.
unfortunately it's a show stopper type repair... sucks too cuz you lose some coolant in the process...
fwiw, it cost me about $800 (not counting the new alternator) from my Indy.