I always smile when I read this stuff:
DDRB = 0x00; // make PORTB all inputs
DDRD = 0x00; // make PORTD all inputs
It's like 0x00 is a "more powerful" form of zero. The concept is nicely sent up here:
In addition to the normal zero, there is the double zero, that's for where things are really zero. This one's pretty zero (points to the single zero) and this one is absolute zero (points to the 00 key).