From ComputerWorld:
What if there were a giant database that contained your hidden insecurities, embarrassing medical questions and the fact that you still think from time to time about your high school girlfriend? Well, such a data store does exist — if you’ve ever plugged such private topics into a search engine.
The fact is, search engines such as Google, Yahoo and Microsoft Live Search all record and retain in their vast data banks any term that you query, in addition to the date and time your query was processed, the IP address of your computer and a cookie-based unique ID that — unless you delete it — enables the search engine to continue to know if requests are coming from that particular computer, even if the connection changes…
The upshot: If someone were to ask one of these search engine companies to produce a list of IP addresses or cookie values that searched on a particular search term, they conceivably could. Or, conversely, given an IP address or cookie value, the search engine firm could produce a list of terms searched by the user of that address or cookie value.