Canine Vestibular syndrome was the first thing that sprang to my mind as well.
The good news is that dogs can overcome this quite well. Your dog's gait may never return to normal, but they do learn how to compensate for their loss of equilibrium and it doesn't seem to affect them too badly for too long, especially the very elderly dogs since they move considerably slower anyway.
One thing you may notice as your dog learns to deal with the loss of equilibrium is that his front feet will step in double time to his back feet. In other words, each front foot will take two steps to each back foot's one step. If you look at the front feet and then the back, it looks like two different dogs walking! The brain is a totally amazing organ that can learn to overcome an awful lot of adversity and this is a great example of that.
I would, of course, take your dog to the vet to have him/her examined to see what the vet says, but my guess is that CVD is going to be the diagnosis. Please check back and let us know what the vet says and also let us know how your dog is doing.
Ghilly
Google "Canine Vestibular Syndrome/Disease" and see if it may fit what your seeing......Karla