While there are many changes in store for the arena in patch 5.4, there are still no development plans for a spectator mode according to EU community manager, Nakatoir.
The latest blue post on the matter reiterates much of what we've heard before -- that yes, a spectator mode would be awesome but would require great number of resources to develop. Thus it's not currently in the cards.
Given the incoming arena and PvP changes slated to arrive in patch 5.4, one might expect the demand for a spectator feature to increase, but who knows for sure? What do you think? Would having spectator mode increase your interest and participation in arena? Do you think the WoW eSports community would grow if there was a dedicated spectator mode like the ones found in StarCraft 2 or League of Legends?