Blizzard announced during Blizzcon 2011 that they do plan on making most (if not all) vanity pets account wide in the next expansion, but other than that they haven't released anymore information or details. It's still unknown what "account wide" means (will companions be accessible only for toons on the same server or across all servers on a single WoW account and what about multiple factions?), and what the limitations will be on the pets (will certain pets NOT be account wide for whatever reason?).
Personally I hope for cross faction, account bound pets (either way for "account bound" is fine by me
) so I can finally have an alliance druid with a horde moonkin hatchling