Here is a snapshot of the rising income inequality and extreme concentration of wealth in the US.
Here is an enquiry into the reasons for the widening rich-poor gap in the US.
Uwe Reinhardt has this excellent post on inequality and widening, albeit not-salient, role of Government in American lives.
Edward Gleaser, Alberto Alesina and Eduardo Porter believe that racial, ethnic, religious and linguistic diversity has been responsible for the much lower support for redistribution of income and social welfare programs in the United States compared to homogenous Europe. This may also explain the success of left-wing parties in Europe relative to the US. See also this and this.
Update 4 (22/7/2010)
Snapshot of income inequality in the US. See also this.