Good news, Microsoft has announced a license change to the Office Communication Server (OCS) 2007 for the MSN PIC (Public IM Connectivity) license, which is needed to communicate and federate with the public MSN/Windows Live network from your OCS environment. The MSN PIC license is now free is you have an active Software Assurance (SA).
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:
Thanks to Jan for the info.