Minor Power

With Felhunters losing almost all of their innate resistance, as well as Master Demonologist being changed (read: nerfed), almost all classes are on equal footing as far as spell resistance and penetration is concerned. A couple of the rare exceptions to this are Arcane Mages using Mage Armor in conjunction with Arcane Shielding and Magic Absorption to attain 140 resistance to all schools, and Frost Death Knights with stacked Acclimation to attain an additional 150 resistance to all schools.

That said, the new max rank of Mark of the Wild talented with the Improved Version that many druids will have in Arena from now on, provides 76 resistance to all schools of magic. Knowing this, if you are a casting class, which includes caster hybrids (with the exception of Paladins because holy resist doesn't exist), you'll want a minimum of 80 spell penetration to counter this because there will definitely be an abundance of druid teams of all specs and forms. If you are a class that also casts shadow-based spells (priest, warlock, and even death knight), you'll want to shoot for 130 penetration instead to counter a Priest's Shadow Protection buff.

Your best bets to do this are using the new Stormy Gems and/or the Spell Piercing cloak enchant.

This enchant appears to have changed in my chant list in the Mists Beta to Enchant Cloak - Minor Power. Permanently enchant cloak to increase PVP POWER by 28. Requires lvl 60 or higher item. Uses same amount of Infinate dust.

The higher level version that uses 3 hypnotics is also changed, to Enchant Cloak - Lesser Power, giving 56 pvp power to item level 300 or higher. I have not seen either of these enchants listed on wowhead so far.

Paladin auras are 130 resistance, so you'll want 130 spell pen as a mage(fire/frost obviously) and as a hunter(survival - explosive shot; all specs - traps), too. As well, every PvP specced Frost Mage(and rarely, imp. CS/Fire PvP spec) is going to have Magic Absorption, and likely will be using Mage Armor, so you'll need 120 to get past that.