The TCG and even "no longer available" pets are all cage-able (for now), but the Blizzcon and CE pets (including the SC2 CE pet) are not.
Here's a list of pets that I've found are currently NOT tradable on the beta (I don't have access to the WoW CE pets, so I can't say for sure whether or not those are tradable, but I'm guessing they are not):
Cenarion HatchlingI don't expect this to be the final list, and developers could change their mind at any time. Things will probably continue to change and shift around as more testing goes on, so we'll just have to sit tight and wait to see what the final list of cage-able/tradable pets will be in MoP.
Soul of the Aspects
Wind Rider Cub
Still, it's a bit strange. Maybe the pets listed above require special coding to make them cage-able? Or perhaps they're just really stubborn and don't like being cooped up in a cage. :P