A year ago, Google began labeling hacked sites and sites with malware as sites that may compromised in the search results snippets.
Yesterday, Google's Matt Cutts announced on Google+ that Google has expanded that feature. Matt said the change they just launched will "expand our [Google's] coverage of labeling search result pages."
We just launched a change which will expand our coverage of labeling search result pages that we believe have been hacked or compromised. We initially rolled out this type of labeling just over a year ago (See for more details). We aim to continue working to notify webmasters of their site issues and alert users to avoid these compromised sites in our search results. As a reminder, if your site receives this label in our search results, be sure to clean up your site and file a reconsideration request. Here's info on how to do that.
Lastly, after you register your site in Google's webmaster console at http://google.com/webmasters/ don't forget to enable email alerts so that you'll get emails about messages like this.
Forum discussion at Google+.