The other day I was doing Dire Maul (East) with Tsunade and Amalgam. We heard that The Razza was in the arena, so the three of us went out to kill it.
When we charged in to attack it we found another group ready to attack. We decided it would be more fun to kill this other group, and then The Razza too. We rushed the healers of the other group, and at some point The Razza started attacking our group! After we made sure that the other group was dead, we engaged The Razza. Just as we finished, the other group came back and rezzed. One of their members started attacking Tsunade, but we quickly killed him and the others ran away. We got some nice leather shoulders and an orb of deception for it. It was also lots of fun.
Some people think its wrong to kill your own faction in the arena, but it seems to me that its the entire point of the arena. You jump in to kill a mob, and then ANYONE can come along and kill you, horde or alliance. Ah well, I had lots of fun.