At the recent Ridge bloggers' tasting there was a fairly interesting discussion about the relative merits of cultured and 'wild' yeasts. Ridge is one of those wineries that never adds yeast, relying on the fruit to ferment on its own, with yeasts from the environment. The debate mainly centered around whether those yeasts were endemic to the vineyard or the facility.
My personal view on this is that there's a strong correlation between the use of 'wild' (or 'natural') yeasts and quality wine, but that (as 99.7% of statisticians will tell you) correlation doesn't imply causality. Or, in other words, just because vintners who use wild yeasts typically make great wines doesn't mean that using wild yeasts will automatically result in a better product.
I was planning a blog post on the subject, only to come across an interview with the man himself - Paul Draper on Alice Feiring's blog. Clearly he can explain these things way better than I can. It's a good read; I especially like the sign-off quote:
In California for at least the last ten or fifteen years we have heard that the wines are now made in the vineyard. What is not mentioned is that in most cases they are then re-made in the winery.