Sorry for the delay, meant to post this earlier...
Gunnar has posted a good entry giving some background on the requirement for WS-Trust.
Federated security is becoming more important as we move up the abstraction layer in software, and think more about assembling applications more and more from reusable services hosted in disparate software environments.
As interoperability specialists, we have been aware of this requirement for quite some time. In fact I contributed to the original IONA security architecture definition about 5 years ago. It has evolved considerably since then, and of course we will continue to evolve it toward standardization as these new security specifications mature.
We have one really great example in production (unfortunately do not have permission to mention the customer name) that federates Windows Active Directory with RACF on the mainframe, creating a single sign on solution based on Kerberos.
ps Like the George Clinton analogy!