Good article. Suggestion: When you post an article like this to hacker news it would help to include the programming language in the title, like "Getting rid of NULL (in PHP)". I wasn't aware that this is a PHP-related post until I've read the first bunch of paragraphs until you actually mention "PHP".
I'm not sure that's necessary, it's not a PHP specific post. The author is just using PHP to demonstrate his point. Most, if not all object oriented languages should be able to replicate this functionality.
