Many of my clients have seen messages like this as well.
Dear site owner or webmaster of ---,
We've detected that some of your site's pages may be using techniques that are outside Google's Webmaster Guidelines. Specifically, look for possibly artificial or unnatural links pointing to your site that could be intended to manipulate PageRank. Examples of unnatural linking could include buying links to pass PageRank or participating in link schemes.
We encourage you to make changes to your site so that it meets our quality guidelines. Once you've made these changes, please submit your site for reconsideration in Google's search results.
If you find unnatural links to your site that you are unable to control or remove, please provide the details in your reconsideration request.
If you have any questions about how to resolve this issue, please see our Webmaster Help Forum for support.
Google Search Quality Team
Clearly suggesting Google is targeting sites that has outgoing links.