[headbirth]

I was wondering if it's possible to deauthorize a PowerBook I no longer own. Does the deauthourizing have to be done from that machine?

[WOPR]

I was wondering that too as I forgot to de-authorise the one I'm about to sell. I fear I'll have to set it all up again though to do it

[TETENAL]

You can not de-authorize a single machine without physical access to it. That is obviously since otherwise Apple couldn't enforce the maximum of 5 machines authorized at the same time (I leave it as an exercise to you why that is so).

Once you have used up all your 5 authorizations, Apple allows you to de-authorize them all at the same time (without physical access). You do this within your iTunes account screen in the iTS. You can do this only once a year though.