One of my favorite bits of acting training has been learning accents, not in the least because it dovetails nicely with some of the linguistic anthropology I studied back in the day. Really, it’s those times where deciding to study anthropology and theater really pay off.
Despite such ardor, I couldn’t tell you when us treasonous colonials gave up our British accents, but Matt Soniak and the ever-intriguing site Mental Floss are here to fill that need (at least on a basic Internet level).