Jeez Garry, what were you doing for that to happen?
Garyj,
It may not have necessarily been one thing. It was not the whole site(s) that got dropped. Certain pages bombed for certain terms (the high exact monthly search terms). Others with lots of on page tweaking but few backlinks actually did slightly better.
The drop probably represents the competitiveness of the keyword (hence three pages for a localised site and sixty pages for a worldwide spread site).
Luckily some things I did with my own sites were not done to clients' sites. These have been untouched
I did not get an "unnatural linking" message in Webmaster Tools and the sites did not get "Sitewide" penalties (some pages still rank quite well for less competitive terms). It is probably due to some factors carrying far less weight than they used to.
As Wynne says, if you do to many things "right" you get whacked!