YOU ONLY LIVE ONCE...MAYBE TWICE!
Wow, haven't been here in a while :P Ok, so I've been thinking right, and being a medical student and all, I think I have a pretty good idea of how the human body (in terms of physiology in relation to gross anatomy) should work under ideal circumstances. Any changes to the regular morphology or function of even the tiniest part of the body (DNA sequence) would alter the way the body works, and since the body won't be at "optimum", it wouldn't have the capacity to function for a very long time, if even at all. So, considering that the body of a zombie is simply the human body with a couple changes to the morphology and physiology, I'd say that a zombie won't live very long. If it's a change in the functioning of the brain, central or peripheral nervous tissue, there's no way in which nerve cells won't possibly die, even if at a slow rate. eventually, the positive feedback mechanisms would completely degenerate the tissues, and then there's infarction and necrosis. That's why I conclude: zombies won't have long life spans.
A horse stepped on my foot today. Ouch.