My PyCon 2008 posts included an item "German, American, or Other" with a list of core python developers (anyone with subversion privs) and a guessing game on nationality. I just updated the post with some statistics on how many Americans come from German immigrants (hint: lots) but I'll repeat the update up top.
German ancestry is dominant** in 17% of Americans or about 50 million people. You can be 1% German and have a German last name or 99% German and have a Spanish last name but if people mate randomly* the number of people with German last names would be at least as high as the number of people who self-report that they are mostly of German ancestry. There are 80 million Germans and somewhere near that number of Americans with German last names so this quiz becomes somewhat less surprising in hindsight.
* and mate randomly they do. The urge to mate is much stronger than any weak favoritism for ethnic continuity. In the US the idea that like-marries-like doesn't last for two generations (see "My Big Fat Greek Wedding"). And the idea is weakest in the majority (aka white people). Growing up I was amazed that the O'Dea kids down the street were 100% Irish. They weren't recent immigrants so the only thing in my mind was: wow, what are the odds?
If you want another anecdote, here's one: my father's father (who's last name I have, obviously) did not like the Pennsylvania Dutch ("Dutch" being the popular corruption of "Deutsch") . He was a 3rd generation New York German but he didn't care for the country bumpkin Pennsylvania Germans who got on a Westward boat 10+ generations before his own ancestors. His bias was entirely theoretical and limited to jokes and stereotypes. He didn't consider my mother (100% Dutchy) or any of her family as "Dutchy" because they were doctors, lawyers, businessmen, and engineers. He was encouraged to not bring it up and everyone else was reminded that he didn't really mean it. "Etlay itway opdray" is our family motto so this was rarely a problem.
** German ancestry is a plurality but thankfully English/Scottish law is dominant. At the time of the Constitutional Convention in Philadelphia, Pennsylvania the dominant language in Pennsylvania was actually German. Those German speaking Americans were not parochial, however, they voted to make English the official language of the state.
NB, if you ever read a story about harsh race relations in America please take my grandfather's story into account. My grandfather's rhetoric far exceeded reality. His words (accurately quoted!) would have been an indictment but were far different than his beliefs in practice.