When logged into any Goolge service (e.g. Gmail, Google Reader, etc...), you search results will be determined by your previous searches. This has not only raised concerns of privacy, but SEOers are concerned that their job will become untenable. As Search Engine Round Table reports:

The Google Blog announced that they will be adding personalized search results by default for any person signed in to their Google Account. That means if you use any one of the following Google services Gmail, Orkut, AdWords, AdSense, Groups, Alerts, and so on... you will automatically be shown search results tailored to your past searching history. [...]

There are two clear concerns here. The most obvious is the privacy concern, but that is for a different blog. The second concern is the search engine optimization area. How can one optimize a site for a search results environment that is constantly changing, not just based on an algorithm that Google updates often, but based on personalized preferences and behavior?

Yes, Google is tracking your search history, just login and click here to see what they have tracked.

The Complete Article offers additional insite with comparative screen shots of what a search term yields when someone is logged in and logged out.