And last but not least, you can now place pets in your Pet Journal into a pet cage.
It's bugged at the moment and when you use the pet cage it will teach the Hyacinth Macaw instead of the companion that's listed on the cage.
One thing to note is that because the spellbook companion tab and the Pet Journal are still two independent collections, removing a pet from the Pet Journal (by placing it into a pet cage) does NOT remove it from your spellbook.
This complicates things because if you want to replace the pet (you just stuck into a cage) in your Pet Journal, you'll need to use a character that hasn't learned that particular pet yet. Otherwise, you'll get an error message stating that you already know the companion and cannot re-add it into the Pet Journal.
Simply knowing the spell for the pet also won't allow you to re-add it into the Pet Journal. You have to actually learn the spell from the pet item.
For example: I used a low level troll warrior to place my Albino Snake into a cage. It was removed from the Pet Journal completely (on all my toons, both alliance and horde). When I logged onto my main character, a nelf druid, and purchased another Albino Snake from Dalaran, I couldn't use it because my druid already had that pet in her spellbook companion tab. In order to replace the snake in the Pet Journal, I had to send the pet to a toon that didn't already know it. Once my draenei shaman learned the Albino Snake, it popped up in the Pet Journal again.
Like I've said before, this is all still very new and there are bound to be bugs and inconsistencies. Blizzard is still working out the details, so hopefully this is something that will just get sorted out as the beta progresses.
UPDATE: The pet cage can be traded and as far as I can tell, you can put pretty much any pet in your Pet Journal into a cage. Even the rarer pets that are no longer readily available such as Grunty. Since the journal and the spellbook are two independent collections at the moment, you can still summon a Grunty once you've sold/traded the one in your Pet Journal. But you have to use the spellbook to summon it and it will not have a unique name or be renameable.