Centennial District Authorization Act of 2008 - Establishes in the City of Philadelphia, Pennsylvania, the Centennial Historic District (District).

Authorizes the Secretary of the Interior to make grants to, or enter into cooperative agreements with, state or local government agencies, nonprofit entities, and any leaseholders to administer the District. Directs the Secretary to: (1) prepare a plan for the development of historic, architectural, natural, cultural, recreational, and interpretive resources within the District; and (2) provide financial assistance for capital improvement projects in the District.