There are two reports, I believe from the same individual, about being prompted via email from Google that Google Webmaster Tools detected doorway pages on his site.
Dear site owner or webmaster of http://example.com/,
We've detected that some of your site's pages may be using techniques that are outside our quality guidelines, which are available here: GOOGLE GUIDELINES PAGE Link
Specifically, your site may have what we consider to be doorway pages - groups of "cookie cutter" or low-quality pages. Such pages are often of low value to users and are often optimized for single words or phrases in order to channel users to a single location. For more information about doorway pages, see: http://www.google.com/support/webmasters/bin/answer.py?hl=en&answer=66355
We believe that doorway pages typically create a frustrating user experience, and we encourage you to correct or remove any pages that violate our quality guidelines. Once you've made these changes, please visit GWT Link: to submit your site for reconsideration in Google's search results.
If you have any questions about how to resolve this issue, please see our Webmaster Help Forum for support: Google Support Forum Link Sincerely,
Google Search Quality Team
Google defines a doorway page as a:
Doorway pages are typically large sets of poor-quality pages where each page is optimized for a specific keyword or phrase. In many cases, doorway pages are written to rank for a particular phrase and then funnel users to a single destination.
Whether deployed across many domains or established within one domain, doorway pages tend to frustrate users, and are in violation of our Webmaster Guidelines.
Google's aim is to give our users the most valuable and relevant search results. Therefore, we frown on practices that are designed to manipulate search engines and deceive users by directing them to sites other than the ones they selected, and that provide content solely for the benefit of search engines. Google may take action on doorway sites and other sites making use of these deceptive practice, including removing these sites from the Google index.
Have you ever seen one of these?