In a fantastic example of being at least three years behind the average year 10 pupil, check this discussion out about how the everyday full stop has acquired a negative tone. I don't know if this counts as semantic change, more a kind of gaining semantic significance when once there was none. It leads into a neat discussion of how all sorts of punctuation is acquiring or changing meaning, and is worth a look simply for how quickly the comments section descends into rowing, with people threatening icepicks in eyes within a few lines. Remarkable.
