This was one of the first things I recall learning about Oracle when I had my first internship dealing with it. It boggles my mind that you can work with Oracle for more than a week and not run into this.
I would even say I've worked with some top-notch DBA's, and I've at times been insufferable in my defense of NULL as a concept, and the databases in question have dealt with a whole lot of data and made a whole lot of money -- yet somehow I didn't know this.
Yay HN. :-)