In town for Tuesday's UN General Assembly speech, (Credit: AP)

In town for Tuesday's UN General Assembly speech, President Barack Obama visited David Letterman in the final lap of an exhaustive tour of TV studios from here to Washington, D.C. (Sept. 21, 2009)

Barack Obama appears on the 'Late Show'

President Barack Obama goes on the 'Late Show' with David Letterman.

In town for Tuesday's UN General Assembly speech,
(Credit: AP)

In town for Tuesday's UN General Assembly speech, President Barack Obama visited David Letterman in the final lap of an exhaustive tour of TV studios from here to Washington, D.C. (Sept. 21, 2009)

David Letterman motions towards White House press photographers
(Credit: AP)

David Letterman motions towards White House press photographers taking pictures of his interview with President Barack Obama during a break at a taping of CBS' "Late Show with David Letterman." (Sept. 21, 2009)

President Barack Obama is pictured with host David
(Credit: AP)

President Barack Obama is pictured with host David Letterman during a break at a taping of CBS' "Late Show with David Letterman." (Sept. 21, 2009)

President Barack Obama is pictured with host David
(Credit: AP)

President Barack Obama is pictured with host David Letterman during a break at a taping of CBS' "Late Show with David Letterman." (Sept. 21, 2009)

President Barack Obama jokes with David Letterman during
(Credit: Getty Images)

President Barack Obama jokes with David Letterman during a taping of "Late Show with David Letterman." (Sept. 21, 2009)

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President Barack Obama jokes with David Letterman during
(Credit: Getty Images)

President Barack Obama jokes with David Letterman during a taping of "Late Show with David Letterman." (Sept. 21, 2009)

President Barack Obama joins late-night talk show host
(Credit: AP)

President Barack Obama joins late-night talk show host David Letterman during a break at a taping of CBS' "Late Show with David Letterman." (Sept. 21, 2009)

President Barack Obama jokes with David Letterman during
(Credit: Getty Images)

President Barack Obama jokes with David Letterman during a taping of "Late Show with David Letterman." (Sept. 21, 2009)

President Barack Obama is seen during a taping
(Credit: Getty Images)

President Barack Obama is seen during a taping of "Late Show with David Letterman in Manhattan. (Sept. 21, 2009)

President Barack Obama is pictured with host David
(Credit: AP)

President Barack Obama is pictured with host David Letterman during a break at a taping of "Late Show with David Letterman." (Sept. 21, 2009)

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President Barack Obama is pictured with host David
(Credit: Getty Images)

President Barack Obama is pictured with host David Letterman during taping of "Late Show with David Letterman." (Sept. 21, 2009)

David Letterman motions towards White House press photographers
(Credit: AP)

David Letterman motions towards White House press photographers taking pictures of his interview with President Barack Obama during a break at a taping of "Late Show with David Letterman." (Sept. 21, 2009)

President Barack Obama is pictured with host David
(Credit: Getty Images)

President Barack Obama is pictured with host David Letterman during a break at a taping of "Late Show with David Letterman." (Sept. 21, 2009)

President Barack Obama jokes with David Letterman during
(Credit: Getty Images)

President Barack Obama jokes with David Letterman during a taping of "Late Show with David Letterman." (Sept. 21, 2009)

President Barack Obama is pictured with host David
(Credit: Getty Images)

President Barack Obama is pictured with host David Letterman during taping of "Late Show with David Letterman." (Sept. 21, 2009)

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ADVERTISE HERE
President Barack Obama talks with host David Letterman
(Credit: AP Photo/Charles Dharapak)

President Barack Obama talks with host David Letterman during a break at a taping of CBS The Late Show with David Letterman at the Ed Sullivan Theater in New York, Monday, Sept. 21, 2009.

President Barack Obama laughs with host David Letterman
(Credit: AP)

President Barack Obama laughs with host David Letterman during a break from taping The Late Show with David Lettermanon Monday.