Neat graduated circle map projecting US pop in 2050 with graduated circles emphasizing major urban centers: http://www.rpa.org/america2050/images/2050_Map_Megaregions_Influence_150.png
Very nice interactive map from NYTimes of Immigration data from 1880. Has both choropleth and graduated circle variations: http://www.nytimes.com/interactive/2009/03/10/us/20090310-immigration-explore...