Let's take a quick look at one of the more popular 41/20 builds for PvP healing. One of the first things you should notice is that there are no points in the Retribution tree. Although the Retribution tree can be utilized for other specs (mostly those that have to do with dealing damage), there is really no substitution for utility and survival talents that you can find in the Protection tree. In order to examine each tree and how it relates to PvP healing, I'm going to break it up into sections.
For the sake of argument, let's assume we're using the talent build I linked earlier.
Improved Devotion Aura 5/5: This is the better talent out of your two choices in the first tier – even though you won't be using Devotion Aura (more on that later), you can't block with your shield while casting heals, which pretty much makes Redoubt worthless.
Toughness 4/5: Kind of a filler talent for this build, but increases your damage mitigation from armor by 8%, and is generally better than Precision (you're healing, not hitting stuff) for this tier.
Guardian's Favor 2/2: Reduces the cooldown of one of your bubbles by 2 minutes, and increases the duration of Blessing of Freedom by 6 seconds. This is a good, solid talent.
Improved Righteous Fury 3/3: Ignore the part about increased threat from holy spells – the real bonus to this talent is the 6% damage reduction while the spell is active. Just remember to use it!
Blessing of Kings 1/1: More buffing options are always useful, and for party members this increases their dodge rating by buffing their agility by 10%, which means they'll take a bit less melee damage.
Stoicism 2/2: Another solid talent for PvP – this one reduces your chance to be stunned by 10%.
Improved Concentration Aura 3/3: You may find yourself asking why you would take this... after all, regular Concentration Aura prevents spellcast pushback from damage when combined with Spiritual Focus, but the real gem here is that it reduces the duration of Silence and Interrupt effects by 30%, which can mean the difference between getting that heal out in time, and a dead teammate. You should generally use Concentration Aura if you're healing full time in a PvP situation.
Divine Intellect 5/5: Increases Intellect by 10%, which in turn means more mana, more healing, and more spell crit.
Spiritual Focus 5/5: You wouldn't want those heals interrupted by damage, would you? No, of course not – that's why you should take this talent. With Concentration Aura on, there'll be no increase in spellcasting time due to damage.
Unyielding Faith 2/2: Increased resistance to Fear and Disorient? Yes, please. Warlocks especially will try to keep healers out of commission in Arenas. This helps you to give them a hard time doing it.
Healing Light 3/3: No brainer. You're here to heal, and this lets you heal more.
Aura Mastery 1/1: Extends the range of your auras to 40 yards. Since you should be using Concentration Aura, this lets your caster teammates get the benefit of your Improved Aura and gives them the freedom to move a little further away from you if they need to.
Illumination 5/5: Even after the reduction (nerf) to this talent, it's still a solid part of a healing Paladin's build, helping to extend the mana pool.
Improved Blessing of Wisdom 2/2: Somewhat of a filler talent, but still not bad to have if you happen to get into an extra long Arena battle and find yourself strapped for mana.
Pure of Heart 3/3: Increased resistance to curses and disease helps against a few different types of DoTs and pesky debuffs. Especially important since we paladins can't cleanse curses, so a higher resistance to them is definitely welcome.
Divine Favor 1/1: For when you absolutely, positively, need to crit a healing spell, there's no better talent.
Holy Power 3/5: Not the most useful talent for PvP, but more spell crit is always nice. I wouldn't put all 5 points in this one because there's more useful talents later on.
Light's Grace 3/3: After casting Holy Light, this lowers the cast time of your next Holy Light spell by half a second. With this talent, you can throw out your big heal almost as quickly as your smaller one.
Holy Shock 1/1: You likely won't be dealing damage with this one, but it's not bad just in case you need to throw out an instant heal.
Blessed Life 3/3: Gives all attacks made against you a 10% chance to deal half damage. This is terrific for PvP, especially when you're up against a fast attacking opponent like a Rogue.
Holy Guidance 3/5: Kind of a filler, but does increase your spelldamage and healing by 21% of your total Intellect. This is nice because it scales with your gear.
Divine Illumination 1/1: Reduces the cost of all your spells by 50% for 15 seconds. Fantastic for mana efficiency if you find yourself in an emergency situation where you have to throw out many big heals quickly.
This build doesn't focus completely on getting the most efficiency out of your healing, but instead finds a strong mix between healing power and maximizing your chance for survival. It increases your resistance to fear, disorient, stun, curses and disease, as well as decreasing the damage you take and lowering the cooldown on some of your emergency / utility skills. Since you can't heal someone very well if you're unconscious, laying dead on the ground, or running around aimlessly flailing your arms over your head, I'd say that this makes this build very strong for PvP applications.