I completely missed this, which I stumbled across via the Reeves Library blog.

Previously, we only offered Personalized Search for signed-in users, and only when they had Web History enabled on their Google Accounts. What we’re doing today is expanding Personalized Search so that we can provide it to signed-out users as well. 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. —Google