From Matt Cutts at Google:

In the next few days, we’re launching an important algorithm change targeted at webspam. The change will decrease rankings for sites that we believe are violating Google’s existing quality guidelines. We’ve always targeted webspam in our rankings, and this algorithm represents another improvement in our efforts to reduce webspam and promote high quality content.

While we can't divulge specific signals because we don't want to give people a way to game our search results and worsen the experience for users, our advice for webmasters is to focus on creating high quality sites that create a good user experience and employ white hat SEO methods instead of engaging in aggressive webspam tactics.

For more details on what this update may mean for you, read the articles at Search Engine Land or the Google Webmaster Blog.