ta-da! it's true, the baby panther is a quest reward from the new (ZG?) quest chain in patch 4.1. this may or may not change once the patch goes live.
the quest that rewards the panther cub is called "some good will come", however, to reach this quest you must complete the pre-requisite quests. the chain starts in stormwind/orgrimmar which leads you to stranglethorn vale (the other seperate, and i'm guessing ZA-related, quest chain takes you to ghostlands). it's a very linear quest chain, so it's hard to miss. if you're being sent to kill a black panther mob named mauti, you are on the right track to your panther cub.
"the panther cub looks at you with a look in it's eyes that says, "will you be my new dad?" - awwww! *insert mushiness here*
one thing to note is that at the time of this writing, the panther cub item is NOT bind on pick up. again, this could change once patch 4.1 goes live.
much like his two cousins, the panther cub has the same model, idle animations (sit and lying down), and on-click vocalizations.
i can't wait to complete this quest and obtain a cute kitty on live servers. of course i'll be his new mommy! who could resist those adorable wide eyes? :P