Google will be personalising search engine results which basically means if you use amazon a lot then amazon pages will get a boost within your search results. From what u have read you will get this automatically and have to actually opt out.
This does explain some of my recent results but as far as i can see does not bode well for the likes of myself who like to check organic listing positions for set keyword combinations as part of ongoing SEO work. even if I opt put how can I guarantee that the results are true? Please read more and I willprd than likely blog more in the future on this.