If you don't have an intellectual property attorney retained already, you may want to look into doing so.

Even if the domain name registration is private on the WHOIS database, you should be able to get that information from the registrar by showing cause. Copyright violation is typically one of those "for cause" exceptions to their privacy policies. Once you have their true contact information, send a cease and desist letter to their main offices so they're on notice of their copyright violation and your knowledge of their violation. I know it isn't the end-all-be-all type of solution, but that may at least give you a start.

Best of luck!

EP