It depends on what kind of pvp you'll be doing... For arenas, frost is the way to go - survivability is key and you can really piss off the opposing team by freezing/slowing them. Anywhere else, though, I think it depends on your playstyle... Each tree has a way to slow down your opponents (Slow, Dragon's Breath, etc). Frost gives you more survivability, Fire gives you more damage. There aren't too many arcane specced pvp mages though. I personally spec to frost for pvp. But I know of some arcane mages who love it for pvp and are successful with it.