Throughout the years of doing SEO on my vacation rental site I have searched google millions of times looking for sites to get links from, what sites are having success with, etc. and it always has seemed to me that google was very conissitent in its ranking factors.
Just recently Google started an investment fund, Google Ventures. Just recently they invested money in the largest vacation rental advertising company, HomeAway. HomeAway owns a network of over a dozen sites all offering vacation rentals.
If they continue to invest in companies that gain the majority of their business from the web I feel it is a conflict of interest for them. Naturally they are going to want companies they invest in to show up high in the search results, which they control.