$field_input_name always returns NULL. It doesn't get much simpler than this.

gistfile1.php

<?php
	# Testing without inputs
	function _xprofile_registration_display_field( $field_input_name = 'field_1' ) { 
		$fields = array(
			'field_1'	=> true, 
			'field_10'	=> true, 
			'field_2'	=> true,
			'field_4'	=> true, 
			'field_6'	=> false
		);
		
		var_dump($field_input_name); # Prints NULL - WTF?

		return ( array_key_exists($field_input_name, $fields) ) ? $fields[$field_input_name] : false;
	}
?>

var_dump always returns NULL, did you mean 'prints'? http://php.net/manual/en/function.var-dump.php

I did mean print/echo/output.

Updated.