but Sondjata over at Garvey's Ghost already beat me to the punch with his entry: On These So Called "Black" Presidents
I'd heard this claim before because some of the Afrocentric bookstores I would frequent carried J.A. Roger's book "The Five Negro Presidents". Basically, the claim is that several of the past U.S. Presidents (not counting Bill Clinton) had some amount of African descent and were therefore only "passing" as white. (The candidates are Thomas Jefferson, Andrew Jackson, Abraham Lincoln, Warren Harding and Calvin Coolidge).
Personally I would not want to dismiss these claims without really looking at the evidence but in some cases the racial identification seems more rooted in negative campaigning in a racist society than on solid genealogical information. Furthermore, as Sondjata's post underlines, the fact that anyone could consider any of the candidates "Black" just reveals some of the extremely bizarre implications of the one-drop rule and the American construction of race.
5 Black Presidents by Dr. Leroy Vaughn
DiversityInc: Obama Wouldn't Be First Black President by Aysha Hussain
black presidents (part four)
black presidents (part five)