what if you were a night elf and wanted to pvp against another alliance race?

No information has been released yet really about pvp, we can only guess but I think you will be able to battle other races of the same faction.

You have several options.  You can engage in arena combat that is prearranged or you cna try out a personal duel, both of which are fully consensual for both sides.  The only way I could really see attacking them otherwise is if you had so much reputation with the Horde that you would become attackable in Alliance cities.

pvp is fun