Daniel Drezner assembles several items that refute the conventional wisdom that in the United States the poor get poorer as the rich get richer.
If you care only about income, the poorest percentage of the population made great strides during the late nineties, completely erasing any losses from the previous twenty years.
In my view, it is best to think of the distribution of income as an escalator, not as a pie. People near the bottom today tend to rise over time. They are replaced at the bottom by new immigrants and young families. Drezner quotes an Urban Institute Report that reports on a Treasury Department study which says, "An individual in the bottom quintile in 1979, in fact, was more likely in 1988 to be found in the top quintile than in the bottom one."