I have 3 lvl 80 mages. If you are just going to focus on pvp I would spec arcane and use a cast sequence that spams all instant cast spells. If you plan on pve/pvp I would spec frost. While frost fire bolt spec & arcane does great raid damage you need to watch for procs and cast a bunch of different spells. With frost all you have to do is spam frostbolt.

Here is my spec: Frost 80 http://www.wowhead.com/?talent=of0qcz0cZZAIcckfu0czgbsx

With focus magic it gives all my guys extra crit because they cast it on each other. Torment the weak is great because frostbolts have a snare which increases your damage. Fingers of frost is amazing for critting with shatter. When its up you have a 50% chance to crit just from the talents. One point in improve blizzard allows your blizzard to proc fingers of frost and torment the weak. So if you aoe with blizzard it hurts. I really want to figure a way to aoe with blizzards. I think it will own heroic trash if I can figure out a good way to do it. For boss fights I pop elementals + mirror image and spam away with frostbolt. When the elementals die I use cold snap and do it again. With my crappy gear mages I can do 2k dps on the heroic boss dummy. I expect that to go up once I get them some decent gear. They have mostly quest blues and 70 pvp gear right now.

Ice barrier has saved their asses many of times. Mana never seems to be an issue either. I can spam away a long time.