2h frost pvp - spell pen?

In all blue gem slots? And if so, why? Thanks.
130+ spell pen. 4 gems/cloak enchant. Don't ask why, just do it.
If spell pen is the same as it was before the 4.0 patch, you should aim for around 130 (max) spell pen rating if I remember correctly. Since most of your abilities are physical rather than spells, being a bit shy of that should be ok as well so long as you're ok with the rare Strangulate or other spell resist.

Other than that, you should be gemming for strength.
http://www.arenajunkies.com/showthread.php?t=172752
4 gems + Back enchant = 135

No more, no less.
but at lvl 85 = fail

130+ spell pen. 4 gems/cloak enchant. Don't ask why, just do it.

This... for Unholy too.
At 85 and beyond you'll want 195 spell penetration. You can go over that but don't go under it.

Cloak enchant + 1 normal gem + 1 JC gem does the trick.
Very informative and thanks for the link all.