Google announced in a blog post that it will personalize results for all users, utilizing data from cookies. It will use up to the last 180 days of history about your behavior.
It sounds like they've learned enough from using the histories of signed in users to generalize the technique to a wider audience. They also believe it has enough benefit to push on everyone, despite the fact that it raises more privacy issues. It will make people more aware that Google tracks how they interact with search results over an extended period of time.