Have you noticed the new 'Web History' feature on the Google search page? If you click on it, you'll see a message that talks about personalised search results:

Your search results may be customized using search activity from this computer.

Disable customizations based on search activity.

Sign in or create a Google account to get more personalized search results and manage your signed-in Web History.
In the past, Google only displayed personalised search results if you were a signed-in user. Things have now changed. Unless you opt out, Google will automatically display personalised search results.

Google blog states:

This addition enables us to customize search results for you based upon 180 days of search activity linked to an anonymous cookie in your browser. It's completely separate from your Google Account and Web History (which are only available to signed-in users). You'll know when we customize results because a "View customizations" link will appear on the top right of the search results page. Clicking the link will let you see how we've customized your results and also let you turn off this type of customization.
To opt-out, click "Disable customizations based on search activity." You can just as easily opt back in by clicking "Enable customizations based on search activity."

Many in the SEO community are unhappy with Google's move as their job just got harder. However, some SEO experts believe this change is not something that SEOs should freak out about:

Let us remember, they are not saying that the way they calculate personalization has changed, merely the presentation (ie; logged in/out makes no difference).

Yes, me and the gang in the SEO Dojo will be doing some more testing to make sure… But my suspicion is that personalization levels haven’t changed from the testing we did earlier this year.