Hat-tip to J-Walk for posting a link for this 2004 publication from the U.S. Census Bureau regarding the ancestry of Americans (based on the 2000 census). The map shows county-by-county the predominant location of residents' ancestors.
I would have thought my Norwegian ancestors whould have given rise to more than 1.6% of the 2000 population, but the only place they form the plurality is up on the MN-ND border and a few other scattered counties.
The yellow region of "American" ancestry in KY, TN and Appalacia doesn't represent Native Americans, but rather people whose ancestors came over I think mostly during Scottish, English, and Irish immigrant waves but who now aren't willing to accept any heritage label other than "American."
The bottom table shows the predominant ancestor heritage of residents of this country's ten largest cities.
There's much more information, especially re methodology, at the link.