I wouldn't mind having to solve a captcha once in a while, but I would hate it out of principle I think, "oh, the game is going that way". On another note, spill the beans Overneathe, what fat loot did Blizzard send us for Christmas?
Captcha would be more annoying than anything. The only real time you could prompt it is before you enter a game, otherwise HC players would go berserk having it pop up mid-game. And I already filled out something to prove who I am before-hand, an authenticator. If I have to continually prove who I am beyond that, I'll probably quit from annoyance.