Divine Guardian is a talent in tier 4 of the Prot tree that adds a side effect to Divine Shield that, while it is active, along with absorbing 100% of incoming damage to the paladin, absorbs 30% of damage coming to everyone in the raid within 30 yards. It's basically an extra cooldown you can use when the party/raid is at maximum danger from AoE - Loken, Sapphiron, adds on Grobbulus or Sartharion... Then for other fights or if the AoE danger doesn't happen, it's a cooldown you can use on the tank (and possibly others) during a spike - a boss like Patch who soft-enrages for the last n%, a drake-enhanced Sarth breath, etc. It's better than the Hand of Sacrifice/Divine Protection combo because it's (a) automatic, and (b) raidwide.
It's changing name but not function in 3.1, but the build will be even more viable because there is a new talent on the same tier that significantly buffs Sacred Shield.
The two big negatives are that (1) you lose the 18-20 points in Ret that give you crit and other goodies, (2) there's nothing between Tier 1 and Tier 4 of Prot that are very useful for Holy, so 10 of the 20 points in Prot are "wasted". So for now it's a "trick" build.