A recent hotfix has reduced the health on tanking paladins. Meanwhile, devastate is being hotfixed as well to do 20% more total damage rather than 20% more weapon damage (from 100% to 120% total weapon damage presently). There are also some scattered reports that DK tanks have gotten buffs to their Frost Presence as well, but no confirmation from Blizzard on that yet. Ghostcrawler posted that the change to Paladin health was intentional and that an announcement will be soon, and in the warrior thread said that they can't confirm when the hotfix for devastate will drop.
The culprit for the paladin health reduction seems to be the paladin talent Sacred Duty, although this has, at this time, not been confirmed by Blizzard so far as I can find. As soon as there's an announcement we at WoW.com will of course bring it to you.
Edited to include this link to the official forum post explaining the change. Thanks to ummeiko for the fast catch.