Biogas Production Incentive Act of 2008 - Amends the Internal Revenue Code to allow a business tax credit for the production and sale of biogas. Defines "biogas" as a gas that is derived by processing qualified energy feedstock (i.e., manure of agricultural livestock and other organic agricultural or food industry byproduct waste material) in an anaerobic digester and that contains at least 60% methane and carbon dioxide and trace gases. Provides an increased credit for biogas produced from qualified cellulosic energy feedstock.