Like Nick Patrick, I too assumed that American and English pronunciation became distinct after the Revolution as American English diverged from British English. It turns out, however, to have been the other way around. Contemporary American English is far closer to the common tongue we and the Brits shared at the time of the Revolution – British English diverged shortly after the Revolution as a trendy “prestige” form of English took root in the mother country. More from Nick, and greater detail from The Cambridge History of the English Language. Hat Tip: Courtney.