Ruh-roh! Once adjusted for inflation (CPI-U, All Items), it looks like real gains in household income have been concentrated amongst few over the last four decades. This seems timely considering the debate over the extension of the Bush tax-cuts. Click on the image for a larger version.
Please remember that real incomes trend toward stability, because if everyone's income increased, then prices would also increase, both because labor is an input of goods and services and because rising incomes would shift the demand curve to the right.