Windows Live: Customers with Office Communications Server 2007 R2 Standard Client Access License or Office Communications Server 2007/ Live Communications Server 2005 SP1 Standard CAL license with active Software Assurance (SA) qualify for federation with Windows Live Messenger without additional licensing requirements. Customers who do not meet the qualifying requirement should buy the Office Communications Server Public IM Connectivity license for federation with Windows Live Messenger.
AOL/Yahoo!: Federation with AOL and Yahoo! requires the Office Communications Server Public IM Connectivity (PIC) per user subscription license. The PIC License provides connectivity to both IM service providers. There are no license options for connectivity to only one provider, but administrators can choose which service provider they provision.
Get more info here
http://www.microsoft.com/communicationsserver/en/us/public-im-connectivity.aspx
http://blogs.technet.com/mikkelbn/default.aspx
http://www.microsoft.com/communicationsserver/en/us/public-im-connectivity.aspx
