Do No Harm Act of 2008 - Prohibits a National Heritage Area designation from taking effect until the President certifies that it will not cause an adverse impact within the proposed Area on: (1) agricultural or livestock production; (2) energy exploration and production; (3) critical infrastructure, including the placement and maintenance of electric transmission and distribution lines, and natural gas pipelines; and (4) the affordability of housing.
Requires that the total deferred maintenance backlog in each state in which a proposed Area is located be not greater than $50 million.