Contrary to what the nuts say Obama has produced a valid birth certificate. You can check for yourself at Snopes.com.
To answer the dual citizenship question. From FactCheck.org
The Rocky Mountain News did in fact run an online article asserting that Barack Obama holds both American and Kenyan citizenship. The article was incorrect, and the paper removed the item from the article and ran a correction. The paper's editor, John Temple, formally apologized for the error in an Aug. 15, 2007, column. Neither the correction nor the apology has prevented the column from circulating across the Internet as part of the latest set of baseless rumors that Obama is ineligible to run for president.
But the paper failed to note that the Kenyan Constitution prohibits dual citizenship for adults. Kenya recognizes dual citizenship for children, but Kenya's Constitution specifies that at age 21, Kenyan citizens who possesses citizenship in more than one country automatically lose their Kenyan citizenship unless they formally renounce any non-Kenyan citizenship and swear an oath of allegiance to Kenya.
Since Sen. Obama has neither renounced his U.S. citizenship nor sworn an oath of allegiance to Kenya, his Kenyan citizenship automatically expired on Aug. 4,1982..”