We saw a noticeable drop yesterday as well, which prompted a look into our detailed stats.
-21% from Google
-31% Ask (makes sense when looking at G and B)
Our main keyword losses were 1-3 word phrases (which we still rank #1-2 for) and we GAINED quite a few longtail referrals. For the longtail results that we seem to have lost, we still rank in top 1-3 places for many of them. (firefox, cookies to G blocked, verbatim)
Its been my best guess in the past that when we have seen results like this, its not due to a ranking flux, but the live-search and auto-complete results and the phrases that are populated into the search box as you type. (You know when you type, and the suggest and results just pop in auto-magically?)
It also explains why we sometimes get junk traffic from G... they are just getting the suggest part wrong for our site, or at least wrong in our definition.