@lazer, is it total visitors or search traffic you're looking at? I imagine it's quite hard to see the signal from the algorithm changes as hopefully your site will increase in traffic year after year which might hide any search traffic signal.
I'd imagine that search traffic as a percentage of the total traffic is a better metric.
We run at about 90% search traffic - pretty much standard for our niche.
In your graphic above, I would say that your total traffic, since June, has grown exponentially, away from the search traffic. Have you seen a more pronounced upwards curve (search traffic) in previous years, at this time?
As we are a Technology based forum, I'm thinking that the dip is a combination of our usual Summer drop in traffic (school holidays, weather etc..), accentuated by whatever Google are fiddling with at the moment. We haven't changed anything, so whatever is is affecting traffic is beyond our control. I'm hoping that we'll see an improvement in line with our usual upturn in traffic, later in the Summer.