If you get a message inside of Webmaster Tools notifying you that you could have spammy links, don’t panic. Google is now notifying webmasters of links it doesn’t trust, but that may not mean your site is being penalized.
Evidently, negative SEO attacks are on the rise. This is when a competitor builds a lot of spammy links and points them at your site hoping that you’ll lose search engine rankings and pave the way for their rise. Attempts like this to affect search engine rankings usually don’t work because Google is able to ferret out the real picture in the link graph.
Anyway, Matt Cutts posted this on Google+.
If you received a message yesterday about unnatural links to your site, don’t panic. In the past, these messages were sent when we took action on a site as a whole. Yesterday, we took another step towards more transparency and began sending messages when we distrust some individual links to a site. While it’s possible for this to indicate potential spammy activity by the site, it can also have innocent reasons. For example, we may take this kind of targeted action to distrust hacked links pointing to an innocent site. The innocent site will get the message as we move towards more transparency, but it’s not necessarily something that you automatically need to worry about.
If you do get concerned about negative SEO attempts on your website or if your site is penalized and you believe it was unjust, submit your site for a site review from within Webmaster Tools. Google is usually pretty fair about these things.