Sacred Shield, a Paladin's Level 80 skill, is a pretty neat ability. Absorbing a base damage of 500, this mitigation scaled with spell power at a rate of .75 to 1, so a Holy Paladin with 2,000 spell power would absorb an extra 1,500 damage for 2,000 damage absorption. Unfortunately, Sacred Shield had a nasty spat with the Retribution talent Sheath of Light since the launch of Wrath of the Lich King. This meant that Retribution Paladins, whose only spell power came from that talent, never got more than 500 damage absorption from Sacred Shield.
Patch 3.1 will, uh, patch things up between the two, who have kissed and made up and promise to play nice with each other. This is a previously reported bug, as Dresorull "the Thread Locker" has noted. He also dispels nasty rumors that Sacred Shield was never intended to scale with Sheath of Light, saying that it was a bug from the very beginning and should be addressed in a future patch. He encourages players to test it out in a future build, when Sacred Shield and Sheath of Light make beautiful music together. Neither talent could be reached for comment.