A Google Press Release announcement  reported by TheNewsMarket says that

"…Google has signed a deal with Nielsen to receive TV statistics. Under the terms of the agreement, announced Oct. 24, Google plans to buy demographic data on the types of individuals watching specific television shows. Google indicates the multi-year agreement will benefit its TV Ads division, launched in May 2007. Google TV Ads is an online platform for buying, selling, measuring and delivering television ads. According to Google, the product along with data from Nielsen will enable advertisers to better understand exactly how their campaigns are performing and quickly make changes as they deem necessary. "

"…NEW YORK, NY and MOUNTAIN VIEW, Calif. (October 24, 2007) - The Nielsen Company and Google (NASDAQ: GOOG) announced today that the companies have established a multi-year, strategic relationship. As a first step, the relationship leverages Nielsen’s experience in television audience measurement to bring demographic data to the Google TV Ads™ advertising platform. By combining Nielsen demographic data with aggregated set-top box data, Google can provide advertisers and agencies with comprehensive information to help them create better ads for viewers and maximize the return on their advertising spending."

My guess is Google needed Nielsen data to refine the targeting of the TV Ads to the right audiences.