re: Your mind will wrap itself in knots if you ponder the term 'African American'

I think recent ancestry is used when someone is asked, "where is your family from?" but I have heard absolutely no white person who was born in America use any prefix to describe themselves. As you said, though, that's due to the melting pot. I think that's because it's easy for Europeans of any extraction to just simply become 'white' in America without any national affiliation, which that same effect is why minorities may keep their own heritage closer.