asusplay - 8:30 pm on May 17, 2013 (gmt 0)
One of my sites got one on at the beginning of March and about 10 days later there was a huge drop in traffic, I am talking about a handful of G referrals a day. The effects are site wide and it seems to affect all keywords it used to rank for except some minor ones.
At first I thought of doing nothing as I thought any bad links would just be discounted, but I was wrong. Ive been getting rid of links, submitted a reinclusion request, got told the site still violates the guidelines...so I'm still working on that and will submit another reinclusion request.
Basically I think that notice is an imminent manual penalty.