From Nielsen:

As the debate on health care in the U.S. continues, President Obama detailed his vision for health insurance reform in his second address to a joint session of Congress on Wednesday September 9, 2009. The address was carried live from 8:00PM to approximately 9:06PM on 10 television networks. The sum of average audience for those networks was 31,848,596 viewers. The networks carrying the press conference were ABC, CBS, NBC, Univision, Telemundo, BET, CNBC, CNN, FOX News Channel, and MSNBC.
Viewership to last night’s address was down 39% from President Obama’s first address to a joint session of Congress on February 24, which aired from approximately 9:00PM to 10:30PM.