Can I see any information about which machines are authorized against my iTunes account? 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.
I also have the same problem and unfortunately there doesn't seem to be a way to view this kind of information at present. This is corroborated by several Apple Support Community threads here, here and here. 

As you say, the only option seems to be to either try to figure out which machines are currently authorized and deauthorize them using iTunes (particularly difficult in the case of multiple VM's or machines you no longer have) or deauthorize all and start again.