John -
An interesting topic. Regarding the question about the effect of cow's milk I have run across the following but have not verified:
1) If you drink milk before bed and don't brush the resulting increase in acidity of saliva increases cavities. However, if you consume cheese instead the opposite effect is true. I don't know if this relates to different types of cheese or not but it would be interesting to see some numbers from cultures with homogenous type. ("Hey. What is the PH for Cheez Whiz anyway? ")

2) Regarding soil PH- Tea lovers often express concern about exposure to flouride leached from the soil of the plantations where their favorite brew is grown. But it seems to me that it might be more exciting to look at the state of the PH of the rain at a given site since that would reflect the conditions over time. But we still have a problem with the lack of geographical clustering. There is something that resists in one individual but not in his (seemingly) identical neighbor.

3) As I have noted before, I have some serious doubts about the general assumption that PD is an old problem due to the lack of description of such a destinctive late-stage patient. They should be stiff and rigid, not shaky. The advent of the Industrial Revolution would have greatly increased the acid-rain effect in England and might even have produced great effect within short distances.