Bennett Haselton, the founder of Peacefire.org, started a discussion on slashdot about google manually removing pages from their index. It may also be that google has lowered the rank or added some other filtering. Some examples of such incidences are "offensive pages" and pages with political satire, credit card numbers, or social security numbers. He compares some of these to pages with similar issues that are not censored.

http://yro.slashdot.org/story/09/12/18/1512228/Yes-Google-Does-De-List-Pages-But-When
http://www.mattcutts.com/blog/remove-page-from-google/