In total, last night's VP debate between Sarah Palin and Joe Biden averaged almost 70 million viewers (69.989 million). It was the most watched VP debate ever and the most watched debate of any kind (including presidential debates) since 1992 when current President Bush's dad, Ross Perot and Bill Clinton squared off (that debate also averaged 69.9 million viewers).
Last night's debate was comfortably ahead of the previous most-watched VP debate between Geraldine Ferraro and George Bush in 1984 which drew 56.7 million.
Source: ©2008 The Nielsen Company. All Rights Reserved
According to Nielsen this measured the following networks: ABC, CBS, FOX, NBC, Telefutura, Telemundo, BBC-America, CNBC, CNN, FOX News Channel, and MSNBC from approximately 9pm to 10:30pm EST. Including PBS, would've added around an additional 3.5 million.