Google's Matt Cutts released a video yesterday answering the question; does Google have algorithms for different positions in the search results.
So does position one through three use one ranking algorithm, position four through six use another and so on.
The answer is no, at least with the web organic search results. Of course the ads have different algorithms, the local results have different algorithms, the videos and so on - but the web results do not work that way.
Here is the video:
Forum discussion at Twitter.