I attended a sheep/goat workshop in southern Missouri, presented by a parasite researcher. She said that the studies and her experience consulting with problem herds shows that it does NOT work. One of the most devastated herds she helped remediate was owned and operated by a DE distributor.
I'll look back in my notes for more detail.
Thank you for polite discussion!
Edited to add: I found a Journal article from the University of Arizona, and the research was done double blind and reversal. It didn't have an effect on worms or growth.
http://www.jstor.org/ This is the home page. You have to register to access the articles.
I'm not trying to convince anyone who is currently using it to stop. Just trying to same folks who are looking at it from throwing their money away on unsubstantiated claims. Even Mr. Wheaton says it works fine on fleas, etc., but in the dry state, not when wet.