Dog

Dogs are a passive mob coming in Beta 1.4., and will be used for hunting and as pets. Dogs can be tamed by right clicking them with bone, which will make them perform "Sit" animation. To give dogs order to attack, left click any mob, causing your dog to attack it.

It was also mentioned at the bottom of one of Notch's posts on his blog, that he had an official working model of the dogs/wolves, and had a video on YouTube about it.

Here is that video. Wolfies (work in progress)