AD RMS on R2 -- new Federation Features
AD RMS on Windows Server 2008 R2 adds a really slick feature blogged about here: Group Expansion for Federated Users Prior to R2 to issue a use license to a federated user they need to specifically be granted permissions. With Windows Server 2008 R2 you can create a contact matching the external federated user and then place the contact in the group and then they have the same RMS permissions as that group.