I would comment in Language Log’s comments feature if it had one, but it doesn’t, so here goes:
Geoffrey K. Pullum writes:
bq. Theres a punctuation rule that American publishers follow rather strictly though British publishers do not: when an expression contained in quotation marks falls at the end of a sentence, a following comma or period (though not a colon, semicolon, exclamation point, or question mark) should be moved leftward to fall inside the quoted string. [The entry goes on to discuss automating this on the computer.]
But this is arsy-versy. In fact it’s the British rule, not the American one. Ritter’s Oxford Guide to Style confirms: British practice is according to the sense when placing punctuation relative to quotation marks (German practice is similar), whereas the US practice is that commas and full points (full stops, periods) are always set inside the closing quotation mark.
The Chicago Style Manual confirms this, although it recommends that there should be exceptions to the American rule. The Americans do treat exclamation and question marks in the way the British treat full stops.
LATER NOTE: My mistake – in fact Geoffrey Pullum describes the American rule correctly, as language hat points out in his comment. However, I am not convinced that there is one rule for both BE and AmE, which American publishers follow strictly and British publishers don’t. There are two rules!