For example: I go to a friend's house, download the StarCraft 2 client from battle.net, install the game and login to play with my account. Afterwards, my friend can play offline to his heart's content (as a guest). Is this actually possible?
Yes I think it is possible but this is what the guest cards are good for. And if he likes SC2 he can buy it for himself .
Very interesting. I guess Blizz is just hopeful that online play is attractive enough to convince people to buy. Perhaps Blizz will disable games utilizing the updater if people don't log into a battlenet account occasionally.
