I was reading the reference page for the bitshift operator (>>) and I saw that it says that when the type is a signed byte/int/long, and the number that you are shifting is negative, the sign bit is copied into the lower bits "for esoteric historical reasons". This makes it sound like there isn't a good reason for those 1's to be copied into the lower bits (and obviously there is). Instead of glossing over the details, you might want to include a reference to 2's complement notation and refer the reader to some learning material in case they're interested and want to learn more. There are a lot of intelligent, motivated people using the Arduino, and I think it's important to use every opportunity to teach the details to people who want to know more. These are the people who will one day be designing our embedded products.
-Justin