If you were planning to get your first couple of levels by AoE farming some convenient packs of Wrath mobs, it's time to change your plans! Blizzard recently announced that it has changed the way XP gains work so that you'll only gain 10 percent of normal experience for kills when you're at the level cap for your expansion zone.
This means that level 80 and higher characters will only get 10 percent of normal XP from Wrath of the Lich King mobs. Likewise, level 70 characters will get 10% XP from The Burning Crusade mobs, and level 60 characters will need to go to the Outland to get full XP for kills.
This change likely happened in the recent hotfix, since at that time, level-capped hunters noticed the XP gains of their pets plummet. Blizzard has acknowledged the problem with pet leveling and promised a fix for that soon. Full blue post with details after the cut.