If I am actively billing clients for a service "X" but someone else is squatting on the domain name URL "X.com" would I have claim to it?

There are 2 ways to get a domain name transferred to you - under the Lanham Act for "cybersquatting" or through an arbitration award under the Uniform Dispute Resolution Procedure (UDRP) which every domain name registrant agreed to when they registered their domain name. In both cases, you would have to show that the other person registered the domain name in "bad faith," which can be shown in a number of ways. One is that they registered the domain name AFTER you had trademark rights. If your trademark is federally registered, that is very helpful in showing that you have rights in the mark "X." You can review UDRP decisions at http://www.icann.org/en/help/dndr/udrp/proceedings to see what you would need to show to get the domain name transferred to you. Otherwise you can search for Lanham Act decisions under the Anti-Cyperpiracy Protection Act (ACPA), for example at http://scholar.google.com/advanced_scholar_search.