Pets will require 33% less XP to level up on the PTR, which (I believe) is in addition to the player leveling changes. Pets still, however, don't earn XP from quests (which makes sense, in a strange way, I guess, because you could bring one pet on a quest, and bring another to turn it in), and they don't earn XP on grey kills, either. Mania's testing also shows that pets earn the same XP on both live and test realms, which means pets don't actually earn XP faster-- they just don't need as much of it.
Good stuff. I don't ever plan to actually level up a pet on its own (I have enough trouble leveling up my alts as it is), but it's good to know that the pet I've got coming with my lowbie Hunter will be grabbing XP faster than ever.