From the publishers website:
Athas.org is recognized by WotC as the Official Dark Sun site. Content created on the official website is considered to be derivative work (as it is based on the intellectual property owned by Wizards of the Coast). This means that fan-created add-ons (such as new net books, adventures, etc.) are jointly owned by both Wizards of the Coast and the creator; neither can do anything outside the official website without the permission of the other.