Just a short while ago, Drysc posted the unfortunate news on the WoW Hunters forum: the ghost wolf (aka the Grimtotem Spirit Guide) can no longer be tamed. Drysc says:
Through a hotfix we’ve recently removed the ability to tame a Grimtotem Spirit Guide. Players who have already gone through the trouble of taming the creature will be able to keep them unchanged. This hotfix only removes the possibility to tame this creature from here on.
The removal of the spirit guide has been debated within design discussions for a while now as to what impact the pet and its precedent has on the game.
The unintended nature of the taming, the undead status of the guide, appearance of the wolf in relation to the feel of the hunter class, and the complex processes of taming were all issues touched upon and discussed.
Ultimately the discussions brought us to the conclusion that this should not be a permanent addition to the pet selection available to hunters; however, those that have already tamed them should be allowed to keep them due to their efforts in obtaining one.
Previously, Blizzard had said that they would be leaving the ghost wolf as a tameable creature. Apparently, they changed their mind.
My thanks to Alanonymous for bringing this to my attention.