Google's acquisition of DoubleClick raises many major privacy concerns. Throughout the late 90's DoubleClick was the boogyman of the privacy community. More recently Google has taken on that mantle. The combination creates an information harvesting juggernaut. Google is in a position to see the search terms, and thus focus of interest, of the vast majority of Internet users. Most users start most searches or web expeditions with a Google search. Their logs contain a fairly complete history of the interests of their users going back for years.
DoubleClick has a view of user activity after the search across thousands of websites. Banner and other website ads are not actually hosted on the websites on which they appear. DoubleClick serves the content from their servers, and handles any clicks on the ads. Importantly, DoubleClick can gather your information even if you don't click on the link. Simply viewing the ad is enough for them to cookie you, to gather your IP address, and store that along with the URL you are viewing.
Combined, this enables the creation of a database most searches along with most subsequent web surfing activities. Nearly ubiquitous Internet monitoring by a single entity will be a reality after this merger. Having both the search information and the surfing activity give the answer to both the what and why of a users actions. The merged data is much more powerful than the individual components, and serve to fill in the gaps in each other's coverage.
Ironically, even Microsoft is talking about the privacy risks of this merger. Redmond | News: Microsoft Warns of Google-DoubleClick Danger
The Electronic Privacy Information Center (EPIC) has gone so far as to file a complaint with the FCC.