The U.S. Census Bureau has released The Effects of Government Taxes and Transfers on Income and Poverty: 2004.  From the summary:

The report provides alternative national poverty rates that range from 8.3%, using a more comprehensive definition of income that includes the value of noncash benefits and excludes taxes, to 19.4%, using another definition of income that excludes all government payments and does not deduct taxes. The official U.S. poverty rate of 12.7% was announced last summer.