On the website http://msdn.microsoft.com/en-us/library/dd451003.aspx, we have a step by step guide to publish the Siebel adapter as a WCF Service in IIS and then have MOSS consume the data using that service. In this approach, credentials are pass by MOSS to the adapter using custom HTTP headers.
If instead you want to consume this service from a .net application or any client that wants to pass the credentials using the ClientCredentials object, this is how you will go about doing it. Follow the steps on http://msdn.microsoft.com/en-us/library/dd450994.aspx that gives detailed instructions about publishing the WCF service. There are 2 differences though
1. In the “Configure service and endpoint behaviors” page, for “AuthenticationType” you will need to select “ClientCredentialUsernamePassword” (as opposed to “HTTPUsernamePassword” that the tutorial specifies)
2. In the “Configure the service endpoint binding and address” page, click on the BindingConfiguration (the … following BasicHttpBindingElement) to customize the binding. Set Security Mode to “TransportWithMessageCredential”
Follow the rest of the instructions for publishing the service. You can now connect to the service using a .net proxy application and pass the credentials using the ClientCredentials object.