The Land Management Information Center has compiled a series of ancestry maps showing ethnic/national heritage for each US county, based upon data from the 1990 census. via Metafilter via Crabwalk.

(Scroll down to "ancestry groups in the United Sates")

The data is not always what you would expect. For instance, the highest concentration of people of English decent is in Utah, and Irish ancestry is highest along the east side of the Mississippi.