Joel Carpenter, Director of the Nagel Center for the Study of World Christianity at Calvin College and the author of Revive Us Again: The Reawakening of Modern Fundamentalism, 1930-1946 (Oxford, 1998), has written a thoughtful piece on American Evangelicalism, titled the evangelical complexion.
Carpenter maps the growing ethnic diversity of American evangelicals and discusses the ways in which this diversity has had an effect on the movement's current, and possibly future, concerns. His thesis: American evangelicals are increasingly 'more culturally varied than is commonly supposed'.
I suspect Carpenter is largely right here. Evangelicals are much too often associated with one ethnic group(White Anglo-American), socio-economic status (the middle class), and intellectual tradition (Puritan).