When I just opened iTunes and authorized my Mac against it for purchases, it told me there were two machines authorized against my iTunes account (therefore, one other one). However, I can't think which that other one would be - I've had various virtual machines etc. configured against iTunes in the past. Are there any ways to find out any more about it (hostname, OS, etc.) so I can check out the situation and potentially deauthorize it?
I'm aware of the limit of 5, and I'm aware of the ability to deauthorize all of them and start again. I'd just prefer to keep things clean and not have stray authorizations hanging around if I can.