President Barack Obama jokes with David Letterman during a taping of the Late Show with David Letterman in New York on September 21, 2009. AFP...
Walt Disney Television via Getty Images SPECIAL - "Battle of the Network Stars" - 11/18/78, Pictured, back row, from left: Sparky Anderson, Howard...
David Letterman attends the 93rd running of the Indianapolis 500 at Indianapolis Motor Speedway on May 24, 2009 in Indianapolis, Indiana.
David Letterman, with assistant Stephanie Birkitt, talks to Mario Andretti before the start of the Indy 500 on May 27, 2007 in Speedway, Indiana.
David Letterman, with assistant Stephanie Birkitt, talks to Mario Andretti before the start of the Indy 500 on May 27, 2007 in Speedway, Indiana.
David Letterman, with assistant Stephanie Birkitt, talks to Mario Andretti before the start of the Indy 500 on May 27, 2007 in Speedway, Indiana.
In this handout provide by the White House, U.S. President Barack Obama talks to TV Host David Letterman during a taping of "The Late Show with David...
David Letterman and Madonna visit the ''Late Show with David Letterman'' at the Ed Sullivan Theater on September 30, 2009 in New York City.
David Letterman attends the Washington Capitals vs. New York Rangers game at Madison Square Garden on April 22, 2009 in New York City.
David Letterman arrives at the ''Late Show with David Letterman'' at the Ed Sullivan Theater on August 27, 2009 in New York City.
David Letterman and Madonna tape the "Late Show with David Letterman" outside Ed Sullivan Theater on September 30, 2009 in New York City.
David Letterman and Madonna visit the ''Late Show with David Letterman'' at the Ed Sullivan Theater on September 30, 2009 in New York City.
David Letterman arrives at the ''Late Show with David Letterman'' at the Ed Sullivan Theater on August 27, 2009 in New York City.
Democratic Presidential nominee Sen. Barack Obama stops by the Ed Sullivan Theater to talk to Dave when he makes his 5th visit to the Late Show with...
President of the United States Barack Obama talks to Dave when he visits the Late Show with David Letterman, on Monday, September 21, 2009 on the CBS...
President of the United States Barack Obama talks to Dave when he visits the Late Show with David Letterman, on Monday, September 21, 2009 on the CBS...
President of the United States Barack Obama holds up the dried heart shape potato that an audience member brought to the Late Show two years ago,...
Steve Martin brought along Martin Short during his visit to The Late Show with David Letterman on Monday October 5, 2009 on the CBS Television...
Steve Martin brought along Martin Short during his visit to The Late Show with David Letterman on Monday October 5, 2009 on the CBS Television...
Steve Martin brought along Martin Short during his visit to The Late Show with David Letterman on Monday October 5, 2009 on the CBS Television...
David Letterman attends Montreal Canadiens vs New York Rangers game at Madison Square Garden on January 7, 2008 in New York City.
David Letterman attends the 93rd running of the Indianapolis 500 at Indianapolis Motor Speedway on May 24, 2009 in Indianapolis, Indiana.
David Letterman and Madonna visit the ''Late Show with David Letterman'' at the Ed Sullivan Theater on September 30, 2009 in New York City.
This September 21, 2009 file photo shows US President Barack Obama with David Letterman during a taping of "The Late Show with David Letterman" in...
David Letterman and Madonna arrive together for a taping of the ''Late Show with David Letterman'' at the Ed Sullivan Theater on September 30, 2009...
David Letterman and Madonna tape the "Late Show with David Letterman" outside Ed Sullivan Theater on September 30, 2009 in New York City.
Madonna and David Letterman tape the "Late Show with David Letterman" outside Ed Sullivan Theater on September 30, 2009 in New York City.
David Letterman and Madonna tape the "Late Show with David Letterman" outside Ed Sullivan Theater on September 30, 2009 in New York City.
David Letterman and Madonna tape the "Late Show with David Letterman" outside Ed Sullivan Theater on September 30, 2009 in New York City.
David Letterman and Madonna visit the ''Late Show with David Letterman'' at the Ed Sullivan Theater on September 30, 2009 in New York City.
Madonna visits the ''Late Show with David Letterman'' at the Ed Sullivan Theater on September 30, 2009 in New York City.
David Letterman and Madonna visit the ''Late Show with David Letterman'' at the Ed Sullivan Theater on September 30, 2009 in New York City.
David Letterman and Madonna visit the ''Late Show with David Letterman'' at the Ed Sullivan Theater on September 30, 2009 in New York City.
David Letterman and Madonna visit the ''Late Show with David Letterman'' at the Ed Sullivan Theater on September 30, 2009 in New York City.
Madonna and David Letterman film the "Late Show with David Letterman" outside Ed Sullivan Theater on September 30, 2009 in New York City.
David Letterman and Madonna visit the ''Late Show with David Letterman'' at the Ed Sullivan Theater on September 30, 2009 in New York City.
David Letterman and Madonna visit the ''Late Show with David Letterman'' at the Ed Sullivan Theater on September 30, 2009 in New York City.
David Letterman and Madonna visit the ''Late Show with David Letterman'' at the Ed Sullivan Theater on September 30, 2009 in New York City.
David Letterman and Madonna tape the "Late Show with David Letterman" outside Ed Sullivan Theater on September 30, 2009 in New York City.
David Letterman and Madonna tape the "Late Show with David Letterman" outside Ed Sullivan Theater on September 30, 2009 in New York City.
David Letterman and Madonna tape the "Late Show with David Letterman" outside Ed Sullivan Theater on September 30, 2009 in New York City.
Madonna and David Letterman tape the "Late Show with David Letterman" outside Ed Sullivan Theater on September 30, 2009 in New York City.
David Letterman and Madonna tape the "Late Show with David Letterman" outside Ed Sullivan Theater on September 30, 2009 in New York City.
David Letterman and Madonna visit the ''Late Show with David Letterman'' at the Ed Sullivan Theater on September 30, 2009 in New York City.