Oxford University Press has just published the eight-volume African American National Biography, which was edited by Harvard professors Henry Louis Gates, Jr. and Evelyn Brooks-Higginbotham.
4/15/08 update: My colleague Michelle informs me that the entries from this set are gradually being added to the online African American Studies Center database (which Harvard subscribes to), where they will be regularly updated. For similar information, see also our online subscription to the African American Biographical Database.