Google just announced that it will be putting a notice next to any websites that it believes have been hacked. If your site has been subject to a malicious attack then in the search results, under the title, the phrase "This site may be compromised" will start to display.
In a post on their Webmaster blog, Google announced..."To protect the safety of our users, we show this warning message for search results that we believe may have been hacked or otherwise compromised. If a site has been hacked, it typically means that a third party has taken control of the site without the owner’s permission. Hackers may change the content of a page, add new links on a page, or add new pages to the site. The intent can include phishing (tricking users into sharing personal and credit card information) or spamming (violating search engine quality guidelines to rank pages more highly than they should rank)."
While Google has placed malware warnings in the search results before, the interesting development here is that even without malware, potential visitors could still be warned away from your site. The "violating search engine quality guidelines to rank pages more highly than they should rank" could have a serious impact on sites that have used black-hat techniques to get to the top of the search engines. Any site that Google deems to have broken the rules may not only receive a ranking penalty, pushing it far further down the results, but the stamp next to it warning users away may lead to a much larger drop in visitors.