Source: http://commons.wikimedia.org/wiki/File:N...

The British settlers coming to different parts of America in the 17th and 18th centuries were from different regional, class, and religious backgrounds, and brought with them distinctive ways of speaking. Puritans from East Anglia contributed to the classic Boston accent; Royalists migrating to the South brought a drawl; and Scots-Irish moved to the Appalaichans. Today's American English is actually closer to 18th-century British English in pronunciation than current-day British English is. Sometime in the 19th century, British pronunciation changed significantly, particularly whether "r"s are pronounced after vowels.