If your WCF's are hosted in a web server environment you can do the following:
In your config file:
<system.servicemodel>
!-- this enables WCF services to access ASP.Net http context -->
<servicehostingenvironment aspnetcompatibilityenabled="true" />
</system.servicemodel>
You should then be able to access the client IP using:
HttpContext.Current.Request.UserHostAddress