It's not a result of being logged in. I was logged in, and signed out, and dogpile was still above altavista when I refreshed the page. However, when I cleared my cookies, Altavista and dogpile switched places.

Most likely due to this : http://news.ycombinator.com/item?id=977461

Who knows why Google calculated that my behavior indicates I'd be more interested in dogpile...