Do not really know how x509 works.
If I have a Web Service that needs to authenticate and validate the client, and he uses a ClientCertificate, could he send me some information that I could then validate against?
I do not create the Web Site itself so I can't be sure they would do it right. I don't wan't to allow someone to say to the Web Service "It's okay, I ClientCertificated him and he's good".
You can implement your own custom client certificate validator but its usage will be different based on the way how do you use the client certificate. If you use pure message security your validator will be the only component used to validate the certificate. If you use HTTPS with client certificate (transport security) the certificate will be first validated by Windows (= your service hosting server must trust the certificate) and after that it will be passed to WCF and validated by your validator.