From a July 24 post on The New York Times' Media Decoder blog:

The conspiracy theorists who have claimed for more than a year that President Obama is not a U.S. citizen have found receptive ears among some mainstream media figures in recent weeks.

Despite ample evidence to the contrary, the country's most popular talk radio host, Rush Limbaugh, told his listeners Tuesday that Mr. Obama “has yet to have to prove that he's a citizen.” Lou Dobbs of CNN said that Mr. Obama should do more to dispel the claims. Larry King, also of CNN, asked guests about it, and other media types, including the MSNBC hosts Chris Matthews and Rachel Maddow, merrily mocked the controversy. NBC News even did a segment on the subject.

[...]

“This smear was thoroughly debunked during the election,” said Eric Burns, the president of Media Matters for America, a liberal media monitoring organization.

Remarkably, there is even a reference to Mr. Obama's birth in the “Births, Marriages, Deaths” column of the Honolulu Advertiser newspaper on Aug. 13, 1961. Still, the claims about Mr. Obama's citizenship persist among a small but vocal group, essentially portraying Mr. Obama as a foreigner who has managed to conceal his origins for nearly five decades.

“It's racist,” said Phil Griffin, the president of MSNBC. “It's racist. Just call it for what it is.”