Forum OpenACS Q&A: Google Changes The Rules - Again!
According to their marketing people it was because Google had changed the rules involving sites which redirect with 302's (something to do with page rank fixing according to them). Ours was redirecting the site root to /shop and then the session handler was redirecting again after adding ?usca_p=t to the URL. Apparently Google don't like/cannot handle two redirects like this. So I fixed it by adding a .vuh to the site root to internally redirect to the /shop/index page and I've also modded ec_create_new_session_if_necessary to just return if the user agent contains googlebot, msnbot or slurp.
Sure enough when I checked today they are back up to 4th in the natural search.
I'm just posting this incase anyone else is suffering the same problem.