I think you have to get cages by pet battling for this to work. To pet battle, Mists has to be released.
However, once it's turned on, it'll put the pet into your inventory, where you can sell it or give it to someone or something. --- 'Cake and grief counseling will be available at the conclusion of the Character Battle.' --MetroidSuperfan on GlaDOS' loss. The Topic Killer since 2001
Yeah, it doesn't work until MoP launches. Like Dawn said, it'll just put a pet from your Pet Journal into your inventory for you to trade/sell. It keeps their level/stats intact. --- Thomx like tommicks