Cross Domain Identity

Hi!
 Suppose you had two domains, Domain-A (say a banking domain) and Domain-B (say a new domain under our control)
- we probably have STS security for Domain-B as we have the freedom to choose the best interoperable security
- but, suppose we need to expose some functionality of Domain-B as service to Domain-A,
   and, Domain-A users would login to their Net-Banking-Portal with an SSO (with uid/pw+secure-id number), and want to access the service of Domain-B
   - WHAT'S THE BEST STRATEGY for securing the service in Domain-B while still allowing Domain-A users continue using their existing security ?
   - Suppose Domain-A used OPEN-SSO (or some other SAML/Feedrated/Liberty) and Domain-B used Geneva Framework based STS

This is usually a real-world case, where we're exited to see geneva kind of security framework, but are challenged by existing security systems to interop.

Regards
Satish
Bangalore. India.