Nice thread over at SEO Chat named Is Google really totally automated?, which asks the question "Is Google totally automated?". There is a poll set up and currently 80% responded "Google is mostly automated, say 98%, but some human tweaks have to be added", 11.5% responded "Absolutely! Google Algorithm is 100% automated! No human factors are involved at all!" and the remaining 7.5% responded "Google is just 60% to 70% automated".
Mike Grehan's search engine book has a wealth of information in it. One of the interviews in the back of the book, I believe, contains an interview with some search engine representatives. In one, I believe, Mike asks this question. Do the search engines ever tweak the results to push a very popular site to the top manually. The response, I believe, was yes they do but not often. Google did not admit to this, but it would not surprise me if Google has some Q&A people do this on occasion.