not sure if its legal or not todo so but its publicly available information by looking up all www.*.com and www.*.netwww.*.org and filtering your server IP to know which domains poiting to that IP and listing them.
might want a lawyer to review zone file access agreement and see if they are allowed todo and view it to public or not.
Bashar Al-Abdulhadi - KuwaitNET Internet Services Serving customers since 1997
Kuwait's First Webhosting and Domain Registration provider - an ICANN Accredited Registrar
This is public information, even on dedicated IP, if they want to check the domains inside each and everyone there is still a way. Rather than worry on this, better to work on how to keep the clients satisfied. A satisfied client will not leave your services easily.