I've known them as rotaries for about 20 years. I first saw them in New England (the Boston area and Maine are really the parts of New England I am familiar with).
Roundabouts (rotaries) do exist in the Midwest. I know of one built fairly recently in Iowa and, according to one website, the highest volume roundabout (judged in vehicles per hour) is currently in Livingston County, Michigan.
http://mtjengineering.com/sagamore_rotary.html"Properly designed high capacity roundabouts have proven themselves to perform extremely well throughout the US. There are good examples as widespread as California, Colorado, Florida, Michigan, Maryland, New York, Utah and many others."
You know the slogan: Rotaries, they aren't just for New England anymore.