The first existing instance your mind jumps to is likely Alterac Valley, but this new instance would be different in that the players are not marching towards two separate goals, split up into two offensive and defensive groups, but rather going after the very same bosses and getting in each others way to a certain extent. Obviously it would also be a lot smaller than AV too, and, like Arenas, based from the beginning on teamwork with your friends rather than random groupings of strangers.
Obviously balancing such a battle would be very difficult, and the losers should feel as though they gain something of value even if they don't beat the other team. When I imagine this sort of instance, I envision something like a maze where players can not only meet up at the last boss, but also sneak around and PvP with each other the whole way through. Different sections of the instance could be designed to provide advantages to different sorts of classes, whether melee or ranged, damage or healing, and monsters could be designed to interact with the two groups of players in some very dynamic ways. Perhaps the bosses and maps could even vary a bit from battle to battle to keep everything fresh.
A Blizzard representative showed up in the original thread to say that he liked the idea too, and hopes that someday we might be able to see something like it. What do you think about it, and what elements would you pay special attention to in order to make it work?