Matt Cutts of Google said in his PubCon Keynote a month or so ago that Google would be stepping up their notifications of spam issues and they indeed have.
In early December, Google started sending out notices of doorway pages via Google Webmaster Tools and now webmasters are receiving more spam notifications.
A paid WebmasterWorld thread has reports of Google warning over "unnatural links" and a Google Webmaster Help thread has reports of warnings over "cloaking". Both are against Google's Webmaster guidelines.
Unnatural Links warnings look like this:
Google Webmaster Tools notice of detected unnatural links on [domain]!
We’ve detected that some or all of your pages are using techniques that are outside our quality guidelines, which are available here:
Specifically, look for possibly artificial or unnatural links on your site pointing to other sites that could be intended to manipulate PageRank. For more information about our linking guidelines, visit http://www.google.com/support/webmasters/bin/answer.py?answer=66356&hl=en
We encourage you to make changes to your site so that it meets our quality guidelines. Once you’ve made these changes, please visit https://www.google.com/webmasters/tools/reconsideration?hl=en to submit your site for reconsideration in Google’s search results.
Cloaking Warning looks like this:
Dear site owner or webmaster of bfenthusiasts.com,
While we were indexing your webpages, we detected that some of your pages were using techniques that are outside our quality guidelines, which can be found here: http://www.google.com/support/webmas...er=35769&hl=en. This appears to be because your site has been modified by a third party. Typically, the offending party gains access to an insecure directory that has open permissions. Many times, they will upload files or modify existing ones, which then show up as spam in our index.
We detected cloaking on your site and suspect this is the cause. For example at http://forums.bfenthusiasts.com/ we found:
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For more information about what cloaking is, visit http://www.google.com/support/webmas...er=66355&hl=en.
In order to preserve the quality of our search engine, pages from bfenthusiasts.com are scheduled to be removed temporarily from our search results for at least 30 days.
We would prefer to keep your pages in Google's index. If you wish to be reconsidered, please correct or remove all pages (may not be limited to the examples provided) that are outside our quality guidelines. One potential remedy is to contact your web host technical support for assistance. For more information about security for webmasters, see http://googlewebmastercentral.blogsp...-now-what.html. When such changes have been made, please visit https://www.google.com/webmasters/to...deration?hl=en to learn more and submit your site for reconsideration.
Sincerely, Google Search Quality Team
Note: if you have an account in Google's Webmaster Tools, you can verify the authenticity of this message by logging into https://www.google.com/webmasters/to...overview?hl=en and going to the Message Center.
Have you also received quality guidelines emails or notices in Google Webmaster Tools recently?