This week, Google partnered with Nielsen "to bring demographic data to the Google TV Ads advertising platform. (...) A key benefit of Google TV Ads is the ability to report second-by-second set-top box data so advertisers can evaluate the reach of an ad and only pay for actual set-top box impressions. Advertisers can better understand exactly how their ad is performing and make near real time changes to their TV advertising campaigns to deliver better ads to viewers. Data derived from Nielsen's representative television ratings panels will provide Google TV Ads advertisers with the demographic composition of the audience."
Cross-correlating data across different mediums could make Google AdWords even more powerful. Google knows the users' intentions (from search queries), what web pages they visit (from Web History or AdSense/Doubleclick's cookies) and now has information about TV preferences. While Google can't connect all this data to a Google Account, it could still use it to create profiles and cluster them, irrespective of the medium. Your actions could infer things about yourself, if you connect them to aggregated demographic data. Google also collects personal data through sites like YouTube, orkut, Blogger.
At the moment, Google uses demographic targeting only for offline ads (print, radio, TV) and for AdSense site targeting, but all the data comes from external sources.
Related:
Google Analyst Day 2007 - Ads and Enterprise Overview