Forum Moderators: not2easy
This question should really go to an intellectual property attorney (we don't do legal advice here!), but certainly one thing that he/she would ask is the legal status of your site name - have you registered it as a trademark (in the US, either with the USPTO or your state), or have you at least flagged it with a "TM"? Is the domain name the name of your business, and has that been registered?
Other relevant questions might relate to the uniqueness of the name - is it a common word or words, or a unique name you invented? Is the other site related to your topic and trying to capitalize on your popularity, or does their choice of domain name look unintentional?
Here's a link to ICANN's Dispute Resolution Policies page: [icann.org...]
Good luck!