Effective as of Friday, December 4, 2009 Google has activated mandatory Personalized Search for all of its visitors regardless of whether they are logged in or not.
Personalized Search is a feature where Google tracks your Web History (recording every query you make, every click you make). As you revisit queries and click on Web sites over and over again, Google gradually changes your search results to favor the sites you click on repeatedly.
Until now Personalized Search was only activated for people who were logged in to Google. You can PAUSE Web History at any time AND clear the data that has been stored. This functionality has been extended to users who are not logged in.
Google uses a secret cookie not tied to google.com to track your browser.
This feature has already caused plenty of headaches for SEOs who have had to explain to customers over the phone how to log themselves out of Google or how to turn off Web history. Now people will become even more confused because they won't realize immediately that even while not logged in Google is recording everything they do and adjusting their search results accordingly.
Your rankings reports will gradually look different from the search results you see. Your ability to hold coordinated conversations with people on the phone (or through VOIP) will be impacted.
Admittedly, this will probably help Google fight Web spam better -- because now users will be able to adjust their search results and ignore sites they don't want to visit any longer.
But this will also force people to rethink their Web marketing metrics. The SEO community has already been debating (quietly) the value of rankings reports for the past couple of years. Many of us have insisted that tracking search conversions is more valuable than tracking rankings because the rankings can fluctuate so much.
Now I think people are going to have to take their conversion data much more seriously than before. Ultimately your search optimization should improve as you become accustomed to thinking in terms of tracking search conversions, but this mandatory personalized search may very well impede the discovery of new Web content.
I speculate that, to assist both Webmasters and users, Google may start injecting fresh content into Personalized Search results -- just as it now injects News, Image, Video, Phonebook, Product, and other special search listings into Web search results through Universal Search.
If you want to read more about how this change may impact your search experience and how you can manage it, Danny Sullivan has a nice writeup at SearchEngineLand (complete with screen captures).
Google Now Personalizes Everyone’s Search Results
Jeremy and other mods -- I recommend you make this a sticky thread for at least a month or two.