According to Godaddy .XXX is now open for registration.
Apparently you can also defensive register a domain to stop anyone else registering it.General Availability is OPEN! If you're a member of the adult community and are ready to promote your adult entertainment website, or you simply want to block your domain from being registered as a .XXX, now is the time to act. Visit our .XXX page to get started.
Prices are pretty steep to start with $99 per year.Can I block a domain name if I don't have a registered trademark? Yes. If you are outside the sponsored adult entertainment community and do not have a registered trademark or intellectual property rights, you can apply for a defensive registration to block a domain name from becoming a .xxx domain name. For more information, see Registering .xxx Domain Names.
You also need membership to the ICM Registry before your domain can go live. But if you plan on doing a defensive registration and not have a live site no membership is required.