If they or you choose to fight them yes. If you type /pvp on an rp server, you are pvp toggled. That means that anyone on the opposite faction can attack you wherever you are.
But you are only forced toggled on an rp / pve server when you as alliance would go to a horde city (ie undercity)
On any server with the name pvp in it, if you are in contested or opposite faction territory, you are pvp toggled. Which is some times no fun when you have level 70s in season 3 coming at you at level 35... but they'll get theirs