Whoa. Just read the coolest thing:
When type was handset, a period or comma outside of quotation marks at the end of a sentence tended to get knocked out of position, so the printers tucked the little devils inside the quotation marks to keep them safe and out of trouble. But apparently only American printers were more attached to convenience than logic, since British printers continued to risk the misalignment of their periods and commas.
This was from a great page on quotation marks.
I found it by searching for "m-dash" on Google.
I should send these pages to all the A&E writers...