Vista - security and architecture question, integration with third party

for guys having integrations background :
when you have a server component that need to connect to third party
server to validate user credentials, what you would suggest ?
the scenario is
1. client sends credentials to ProxyServert
2. proxy server send message to third-party web service to validate
credentials
3. third party responses to proxy, proxy responses to client

both my side (Proxy server) and another side (third party
authorization API) could be done in WCF
but it has to be standard-compliant in a case when another third party
need to be integrated.

i understand that WS* (WS-trust etc) should be used by confused about
1) what exactly to use from security side (Kerberos, X509 etc)
2) how to accomplish it with WCF

bottom line : what standards would you use for such type of
integration and how it can be accomplished from WCF side

Thanks much for everyone