This is a very tricky topic to address, because there are quite a few different approaches to multiboxing out there and only some of them goes against our Terms of Use.

It is an issue if and when multiboxing automates gameplay, but as long as the player who is multiboxing is the legitimate account holder on all the involved accounts, and as long as all the characters being played are 100% under the player's control without any kind of automation involved, then we are okay with it.

Edit: Removing the air plane analogy, because looking at it in retrospect, I can see that it wasn't really very accurate. Please look to my next response for further clarification: http://eu.battle.net/d3/en/forum/topic/5825582768?page=2#25

Again, the question of whether or not multiboxing is permitted is a very hard one to address.

You are free to play more than one character at the same time using more than one client at the same time, but each character must be played the same way you would play a single character normally in a single client. This means that one character must not react and perform actions automatically based on actions performed by another character. You can send commands and instructions to multiple characters at once, which is fine of course, as long as each action performed by each character is a result of direct player interaction and player choice.

Multiboxing characters must be controlled 100% by the player!

Edit: Edited the post for clarification purposes.