So how about this.. amphitheaters situated around the map, perhaps just a few per alliance. They are all situated low, and obviously in a bowl so anyone arriving can see who is already there. Upon entering the arena you are pvp enabled. This will be for friendly pvp, duels, training etc..
If the above is unrealistic from a programming view (and I imagine that could well be the case) then just a load in pvp area, dotted about the map, so that guys from the same alliance can throw down once in awhile.