In the case a null argument is passed to a property marked as isRequired:
http://jsfiddle.net/jeanlauliac/0n6snb6b/1/

We'll get a proper warning in the console: Warning: Required prop namewas not specified inHello. (though it should probably be "prop was null" and not "prop was not specified", but anyway)

On the other hand, the default value is only used when the prop is undefined, but not when it's null. Is this the explicitly wanted behavior? If it is, then we should probably make the documentation explicit about it (http://facebook.github.io/react/docs/reusable-components.html), giving the entire responsibility to component callers of safeguarding against null.