They could definitely figure out if a person was multiboxing or not (after all, players can figure it out most of the time just via observation). So if Blizzard wanted to forbid it, they could. People would still need to report it and they'd still need to investigate it, but just announcing it would probably get most of us to stop.

I get the impression that multiboxing is becoming more acceptable to game developers. WoW has shown that allowing it doesn't harm the game. Sure, some people will complain, but way more people complain about other things that they feel are unfair or wrong, and Blizzard ignores them. Game developers are happy to allow play styles that will bring in players without causing other players to leave. And despite the complaints, the people who gripe about multiboxing do not cancel their accounts.

I just ignore the people who try to complain or flame me in the game, and from time to time I enjoy antagonizing the ones who complain on the forums. The whole debate over multiboxing is long over, and we won. So if someone wants to make the same dumb thread again, I reserve the right to poke them with a sharp stick.