Mage
Not too many changes have been announced for Mages yet, although one key point was Blizzard's intention to raise Fire Mages' survivability in PvP. Despite the nerf to Spirit across the board, they're also looking at making Spirit more appealing to Mages. We'll have to wait for more announcements to see how Mages, already dominant in PvP as Arcane burst is still ridiculously high, will shape up in Patch 3.1.

Paladin
Oh boy. Where do we start? Blessing of Kings is going to be baseline, which is a phenomenal change as it's the single best raid buff in the game. It's also the best Paladin buff for Arenas, although the changes to mana regeneration in Patch 3.1 might push Blessing of Wisdom's utility much higher. Exorcism being castable on all targets is a DPS buff, but more notably, it's a 30 yard direct damage spell usable against enemy players, expanding a (Retribution) Paladin's PvP options.

The real winner, however, is Protection. All the changes are excellent for PvE and solve a lot of caster mob problems, but the buff to Judgements of the Just will make Protection Paladins true stunlocking gods in PvP. Even subject to diminishing returns, Protection will now have access to a 10-second spell interrupt and Seal of Justice will become the seal of choice for Protection Paladins in PvP. Furthermore the Shield of the Templar change, which confers a 3-second silence effect to Avenger's Shield and Shield of Righteousness, makes Protection Paladins the most frightening anti-caster class in the game. That's right, even more frightening than a Death Knight. This is right in line with what Ghostcrawler described as making Protection viable for PvP. Because if a stunlocking, cast-interrupting, silencing class with bubble isn't viable in PvP, I don't know what is.

Priest
Divine Spirit being turned into a baseline spell isn't such big news for PvP, seeing as how Spirit's going to get nerfed, anyway. What sounds awesome, however, is the talent that's slated to replace it -- Power Word: Barrier, which is a Power Word: Shield for the entire raid. Discipline Priests have always been the kings of mitigation, and this new spell looks like it's going to be phenomenal in a PvP setting. What's even better for Discipline Priests is the change to Penance, allowing Priests to target themselves with the spell. If you thought Discipline Priests were tough to kill then, they'll be even more frustrating to try and take down after Patch 3.1.

In other news, Dispersion will now remove snares and Shadowform will also mitigate spell damage. Both changes will help make Shadow Priests more viable in PvP, something that Blizzard hoped for but didn't achieve in 3.0. Discipline Priests make up an overwhelming majority of the top rated Priests in Arenas, so expect Blizzard should do a lot more than make tweaks to Dispersion and Shadowform.

By the same token, they've expressed an intent to make Holy more viable for PvP. Whether this means more utility or survivability -- ideally both -- we'll have to find out when they announce the rest of the changes. The buff to Serendipity works both in a PvE and PvP setting, where faster cast big heals are the order of the day. Overall, it looks like they're moving in the right direction.