The problem that prot. warriors have in fights against casters is that your greatest assests (your high avoidance, armor and mitigation) are a total non-factor since thay give no protection against magic damage. Spell reflection is nice, but its high rage cost makes it difficult to have ready when you need it, and some attacks bypass it.
Your assets that you have are Shield Bass (which is hard to use against many casters since most of their spells are instant cast or fast casting), Hamstring, Intercept, Charge (if you can get the drop on them), Concussion Blow, Intimidating Shout (be mindful to stop your auto attack before using), and (if you have the rage to use it) Spell reflaction. Either CB and IS are likely to be negated by the PvP trinket, so burn CB first as it has a shorter stun time and shorter CD. Your high HP will also be an asset.
As far as talents go, Tactical Mastery is a must as you will be stance dancing while fighting casters. Imp. Defensive stance helps some too. Imp. Shield Wall is also great. It's just too bad the it has a 30 minute CD. Finally Imp. Shield Bash will be helpful since it'll stop them from casting for a few seconds even if you mistime your attack.
If your lucky and skilled, you'll win the fight if you get them down to the magic number 19% and can destroy them with an Execute. The odds are against you if they fear, stun or immobilize you (having your own PvP trinket is nice), but if you burn all your cooldowns and time you attacks right, you have a small but fighting chance.
Hope this helps =)