The Arenas have now been setup as Warzones. Basically this means that you will no longer lose manpower when fighting in the Arenas.

Friendly fire is also disabled, so factions can have faction v faction battles without declaring war. If you want to fight against allies or other faction members then you will have to temporarily break alliances or join/leave factions before you can kill them in the Arenas.

Note that joining a faction for a few minutes so you can create a different team to fight in the Arena will NOT give you access to their faction portal. Players can change factions at will to create teams for the Arenas without worrying that they sacrifice their location via portal exploit.