During GDC earlier this month, Notch, the lead developer of Minecraft, tweeted that he had promised a well-known developer that he would add dogs to the game. That tweet came a short time after another stating he had just met Peter Molyneaux (the guy behind the Fable games), and I'm inclined to think that it wasn't mere coincidence. My initial hope was that even if they were committed to adding dogs they would put it off in favor of other additions to the game, such as the "hardcore" option they've been talking about, wherein you'd need to eat to survive.
But a recent tweet from Jens Bergensten, another of the game's developers, strongly suggests that dogs and/or wolves will be added to the game fairly soon. The tweet included a link to a screenshot of the animals in-game (click for full size):
He's stated that he's made progress in the AI of the animals as well. Why I'm assuming that both wolves and dogs will be added to the game is that he's mentioned that there is a separate "aggro" form of the animal, and that he's considering giving them separate textures so that the player can easily tell them apart.
So it would seem that we're getting them soon, regardless of whether I think they should be a priority. Here's my hope for them: most of them should be wolves, and it should be an incredibly challenging task to tame them. If getting a pet dog is simple, then I don't see the point.
It's not as though we need an ally at this point in the game. The way the game is now, it's not too hard to get most of what you need from within the safety of your fortress while making a few mad dashes outdoors when you're running low on a particular resource. What they need to add before allies is a reason for them. This could be done in several ways: the "hardcore" mode I mentioned above that would give you more of a reason to venture outdoors, the ability for hostile mobs to break into your fortress which would increase your need for security, or even some game-generated goals that would put you in harm's way more often. Then we would have a need for dogs.
(Updates 3/17: Jens has confirmed that the wolves will be tamable on his twitter feed. He mentioned off-hand that bones could play a part in the process, but I didn't take it as concrete. He's also said that wolves will "usually" be neutral. Spiders (during the day) are current neutral mobs in the game; they only attack you when provoked. So perhaps something similar for wolves? He also mentioned a "pack" of wolves attacking a creeper, losing four of their own in the ensuing explosion. No idea yet if that means they'll display pack behavior, or if he was just talking about several wolves that were spawned in the same area. Also, attacking creepers doesn't sound like a very productive behavior, so I don't know if that will remain a feature of their AI.)
Jeb's twitter feed offers some additional information: You tame the wolves with bones; the number required is random, but seems to be around 2-3 most of the time. At this point you can have an unlimited number of tamed wolves, but it sounds like they're looking for a way to cap that. Your tamed wolves will attack any mob that attacks you or that you attack in melee combat--as of now, hitting a mob with your bow will not cause the wolves to attack. They're also trying to work out how to handle tamed wolves in multiplayer, the main problem being what happens to them when you log off.)
On that topic of game-generated goals, Notch has mentioned that while Jens is working on the dogs, he's working on in-game achievements. I'm not sure how I feel about that. Sure, it could offer you more reasons to go out into the open, but they might not be substantial ones, and I worry they would take away from the atmosphere (he's mentioned that they'll only be for bragging rights, so that's good, in my opinion). The only example he's given so far was "riding a pig off a cliff," though I can't be sure whether he was being facetious or not when he said it. That would be fun, but that's the sort of thing I'd want for a secret achievement, so you'd be pleasantly surprised to get it if in the course of your game that crazy series of events came to pass; it's not something I'd to actively work towards. Hopefully most of the achievements will complement the tasks players give themselves already, like building a structure that's impressive in some particular way.
Honestly I just don't think there are enough options in-game at this point to have a worthwhile achievement system, but I guess having one in place early could be a good thing. There are just so many other things I think they could be adding to the game at this point.