Belfaire already explained previously that Blizzard's okay with the idea of multi-boxing, whether it's for PvE use or even to compete in the Arena. In the same Eurogamer interview that discusses the new phasing technology, Tom Chilton reinforces that Blizzard is "content to endorse multi-boxing to some reasonable degree."
Eurogamer was focused on the possible abuses of Blizzard's Recruit-a-Friend program, and the speed with which players can create and level new accounts. Chilton says that if someone wants to drop the bucks on a new account -- not only the retail fees, but also the subscription cost -- then Blizzard's okay with that.
J. Allen Brack immediately brings the discussion back to a point I've previously made -- the idea is really to help players get ready for the new expansion. You can bring in a friend or family, and get them to a level where they can play with other friends very quickly. They want new players to be able to "roll a Death Knight and be ready to go."
Now, Chilton did say the endorsement only goes to a "reasonable degree." I suspect if we see vast abuse of multi-boxing in multiple areas of play, the endorsement might quickly be repealed or re-focused. But, for now at least, Chilton has confirmed Belfaire's statement -- Blizzard's okay with multi-boxing.