Props to Erik for the anatomy lesson, and particularly for the insight as to what effect a hit to different parts of the brain might have on a zombie (as opposed to on a living person). I'm going to base my comments on Erik's anatomy analysis but looking towards a tactical application.
I think the basic take-away from Erik's anatomy lesson is, "Shoot a zombie anywhere from the nose up and you stand a good chance of destroying its ability to move." This seems likely based on the diagrams provided, with the caveat that there are a lot of areas that aren't necessarily used by a zombie and so a hit to those areas might have little or no effect.
What I cannot help but thinking of is a line from Return of the Living Dead: "It worked in the movie!" (Referring to NOTLD, when shooting a zombie in the head didn't work.) Obviously speculating on killing zombies is based on extrapolating from a fictional universe but as we've been doing in this thread, it can be done with some logic. Nevertheless, I think it's important to remember that it IS speculation; therefore when the time comes that one of us sees the first zombie coming at us, take nothing for granted.
I would say that, for myself, I plan on aiming at the triangular area encompassing both eyes and the nose, with the points of the triangle being at the outside corners of each eye and at the center of the upper lip. My reason for this is based on the idea of avoiding the thick, domed surface of the skull, which anecdotally I have heard often deflects low-velocity rounds. Also, I have heard this triangular zone, probably analagous to the "T-zone" described above, as the ideal target area on living humans, as well. (Can't recall where I heard this, probably some book on defensive shooting but I don't recall for sure.)
If one or two rounds to this area does not produce the desired effect, I plan on shifting my point of aim slightly higher, to the forehead. It may be for example that from my angle, the rounds are hitting the face in the correct area but passing to the right or left of the cerebellum and brainstem, or that on a zombie it turns out they don't need their cerebellum and brainstem as much as I'd thought. Hopefully if the cerebellum isn't critical the cerebrum is.
After the first engagement, of course, I'll remember what worked, and do that first, next time. Assuming, that is, I survive the first engagement.