Being Windows Live ID one of the existing authentication modes and that has as big advantage the already existing infrastructure provided by Microsoft and with no cost associates, and also making the Single-Sign-On for several solutions that use Windows Live ID as the authentication mechanism, and if we add the fact that there are mode then 420 millions of registered users of Windows Live ID and more then 1 billion of authentications by day we see that this is a well known and with high used form of authentication. Of course we know that this values have to do with the usage of MSN Messenger, but we can capitalize the knowledge and trust from the users in our solutions. But when we look to enterprise solutions there can be situations that need to share authentication to our solutions between Windows Live ID and at the same time the users that exist on our enterprise and that are managed by Active Directory, LDAP or any other (Ex. An Online Store that we want to give customers the possibility of authenticating using Windows Live ID, and the Administrator Users are solutions from our Company, XPTO, that has their users registered on Active Directory.). In this cases what can we do?
The solution for this question is to use Federated Security. For this the Windows Live Platform Team made available a service that provides this and also published this very interesting article on MSDN where you can see how it works and how can we take advantage of it. I strongly suggest this reading.