It seems as if Google has stepped up its use of the Google Notification of Site Removal emails, that we first reported on in September of 2005. A WebmasterWorld thread named Warning from Google about black hat SEO shows a Webmaster or two discussing the emails they received. The thread notes, you need to be careful about fake notifications from spammers and make sure to authenticate the email source header.
It appears that the newly formatted delisting email from Google reads;
Dear site owner or webmaster of yoursite.com/,
While we were indexing your webpages, we detected that some of your pages were using techniques that were outside our quality guidelines, which can be found here: http://www.google.com/webmasters/guidelines.html In order to preserve the quality of our search engine, we have temporarily removed some webpages from our search results. Currently pages from yoursite.com/ are scheduled to be removed for at least 30 days.
Specifically, we detected the following practices on your webpages:
Forum discussion at WebmasterWorld.