MMO-Champion has a translated version of the WoW China Developers Q&A. Two notable questions related to pet trading and Pet Battles were answered. [source]
Will there be any rules about pet trading? Will we be able to trade pets of limited editions (like Panda Cub) or Blizzard Store pets （like Wind Rider Cub）? How about achievement pets?
We plan to allow players to trade Blizzard Store pets excluding Guardian Cub (because it's not BoP and it's unique). Collector's edition and other limited edition pets (like Mini Tyrael or BlizzCon pets) will not be tradable, either.
You said earlier that not all pets can take part in pet battle system. What are the exceptions?
Currently the exceptions are pets with humanoid children models. They will not be included in pet battle system. Nor will the Guardian Cub.
Keep in mind that these answers may differ on outside of China. We're still guessing that balloon pets (and other similar lifeless models) will not not be able to fight.
Pet Battles and Pet Trading Limitations
April 21, 2012