Multiboxing: Playing multiple accounts concurrently using multiple computers or application instances.
First, it should be noted that most major MMO's allow multiboxing, so it is not against the rules. It may be against the spirit of the rules though.
I think the issue of multiboxing falls into many gray areas. For example, using it in WoW PvE seems perfectly fine to me. The thought of running some small group content, by myself, is actually pretty attractive. Since it is PvE, you are not really hurting anyone else, unless you are just being a jerk.
PvP is where I start to get unsure about the issue. Multiboxing seems to offer the advantage of no need for communication and ease of focus fire. That same advantage may also be a disadvantage though, as you are limited to how well your software does in allowing you control of the other characters. I recall Xzin doing pretty well in PvP. Still, I lean towards multiboxing in PvP giving to an unfair competitive advantage.
With more and more MMO's coming out the player population is getting spread out too. MMO's require cooperation, but what happens when there are not enough players and it just gets tedious? Multiboxing is one answer to that, although not necessarily the correct one.
So what do you think about Multiboxing?