Is it against the law to loan game keys? I was going to remove this post, but then I thought about it....
I don't see a problem with loaning a CD key for the weekend. I suppose technically the loaner would legally have to uninstall the game, but I'd be ethically satisfied if the game were simply not played while the key is loaned. And all this is assuming that the loanee is trustworthy and will only use the borrowed key for the same amount of time that the loaner doesn't use the game.