Hacked Sites Notifications Added to Google Search Results
According to Google's blog post, the company uses "a variety of automated tools to detect common signs of a hacked site as quickly as possible." If they note something suspicion, they;ll add the new notification to our search results.
If the site has a Webmaster Tools account, they'll be notified as well; additionally Google will use any contact email addresses found on the site.
Naturally this will affect a site's traffic, and one problem is that Google says it may take days for a hacked site notification to be removed. Fortunately, webmasters can request a review of their site to accelerate the notice removal.