inbound - 11:21 pm on Mar 24, 2012 (gmt 0)
A few observations from the UK. Although we don't have the traffic level we used to (which was enough to spot a trend very quickly) I'm pretty sure that it has hit the UK.
It may be worth looking out for a larger drop in visitors that were logged in to Google - we saw a much larger drop in these in comparison to the overall drop. This may be a symptom of a rollout that is incomplete.
The long, long tail appears to be where we have lost penetration and this has resulted in a 20% overall drop. There may have been a widening of the matching of keywords so that "more trusted" sites that had close matches gained more exposure than less trusted sites with exact matches - that's something I have seen change a few times and it looks as though Google are getting more confident on what they feel they should treat as being close enough to allow a match (being correct on many but also having some very poor results on the margins of their algo - where data is sparse). Google seem to be taking the view that a poorer match from a "better" site is preferable to them.