First I want to say I'm not a 2000+ arena player, however I am fairly expirenced in arenas and have finally settled on a build I like.
My build: (17/0/44)
Disc Tree - 17
5pts in Unbreakable Will
I feel this is much better than Wand Specialization for Tier 1
2pts in Imp Fort
More health ftw
2pts in Martyrdom
This is extremely useful when a pesky rogue/warr/huntard is critting you and you need to dps or offheal.
2pts in Imp PW: Shield and 3pts in Absolution
This is more of a personal preference, however you may prefer 5pts in
Silent Resolve to reduce the chance your DoTs will be dispelled..
1pt in Inner Focus
2pts in Imp Mana Burn
More of a defensive talent, but mana burn w/o it sucks.
Shadow Tree - 44
5pts in Blackout
2pts in Imp SW: Pain
Longer Duration = More dmg.
3pts in Shadow Focus
Filler to get to Tier 3, more useful than other talents.. imo
2pts in Imp Fear
Less CD means more CC. Not to mention it's needed for Silence.
2pts in Imp Mind Blast
Less CD means more CC...
1pt in Mind Flay
Very useful after fear, keeps those rogues/warrs away for a bit longer; keeps huntards from running too far.
2pts in Shadow Reach
Helps with the low range of Mind Flay, ensures you can still do dmg and apply DoTs as far as other pvp casters.
5pts in Shadow Weaving
More of a personal preference, I like to have as many dots on a target as possible.. not to mention this helps the lock on my 3v3
1pt in Silence
Essential for CC
1pt in Vamp Embrace
Don't be selfish... heal yourself and others while you're in shadowform

3pts in Focused Mind
As a pvp spriest, your mana pool will be much smaller than a pve toon.. so the less mana you need to use the better...
5pts in Darkness
More dmg ftw
1pt in Shadowform
More dmg ftw, and it looks cool

5pts in Shadow Power
A lot of high rated spriests skip this talent all together.. but since my arena ratings are still 1550~1650 I am still able to crit the undergeared noobs. Personal preference.
5pts in Misery
Does not require a stack like Shadow Weaving, thus is usually more attractive.
1pt in Vamp Touch
Help yourself and your team out, return mana as you pwn.

That's my build and explanation.. you should change it to fit your play style. +Rep if my guide helped 
JJ