Just yesterday, Google announced that they will display personalized search results based on usage - regardless if you are signed in or not. So this means that they now track the search activity of All 'enabled' (by default) users and display customized search results. They are tracking 'signed-out' users via a cookie linked to the web browser. Users *can* opt out of "search customizations" but i'm confident that a majority of people will not...
This has already raised some privacy concerns over at Search Engine Land, but from an SEO perspective, I see it as another shift in favoritism toward the "white label" web destinations. For example, if you have a niche travel site but the end users mostly clicks on Expedia or Trip Advisor, it is likely that they will be displayed ahead of your site in the SERPs. Even though you might have the more relevant and resourceful website. I'm interested in hearing everyone's thoughts about this announcement...