It does look like the pet battles system will not be included in pre-expansion patch 5.0, however. We don't know the release date of Mists yet, unfortunately, so it looks like you have plenty of time to stockpile companions before being able to battle them.
[via Warcraft Pets]
It's open warfare between Alliance and Horde in Mists of Pandaria, World of Warcraft's next expansion. Jump into five new levels with new talents and class mechanics, try the new monk class, and create a pandaren character to ally with either Horde or Alliance. Look for expansion basics in our Mists FAQ, or dig into our spring press event coverage for more details!