Microsoft Live has a STS for Windows Live ID (aka Passport) running here. Now this is really interesting, particularly in the context of Microsoft's recent move to get the Infocard selector to many platforms. So what is the rationale behind this? Here is my take on this:

ADFS will be the Microsoft implementation of the Enterprise STS. If it advertises iteself now as a ADFS Federation Partner (i.e. a 'trustable' resource for your enterprise AD), you will be able to provide SSO for your customers to log into your extranet. Now the really interesting question is: will Microsoft allow the Passport STS (by explicit business contract) to trust ADFS deployments (maybe for really large cutomers only), thus enabling your enterprise users to SSO into Passport sites?