I tell you something annoying, I had a collection of free music, and songs I paid for on iTunes.
After having a few problems with my PC, formatting it. Afterwards the ones I got free worked fine but the ones I bought I could not use anymore as I had used them on 5 systems or some crap! (of which I didnt, only that the PC had been formatted a couple of times).
Way to influence my future destination for music downloads...
Late reply, but I want to give some help:
"Once you reach five total authorizations, a button will appear in your iTunes Store account that will allow you to deauthorize all your systems, so that would at that time recover the now-useless authorization."
Straight from Apple support. I had to do this once as well.