from the ISA Server Team Blog:
Recently (July 2007), a deployment lab guide for Microsoft® Communicator Web Access 2007 was published on Microsoft Download Center. That white paper will guide you through the configuration of ISA Server 2006 and Microsoft Office Communicator Web Access 2007 (I will refer to CWA 2007 in this post) to allow single sign on authentication. The scenario described in the document uses LDAP to access the Active Directory® and validate user credentials. After that, the user will be automatically signed in to CWA 2007.
However, the entry page that ISA Server 2006 publishes externally is the ISA Server 2006 page, which uses forms-based authentication. This means that we don’t have the Microsoft Communicator Web Access 2007 interface published. Some customers might want to offer their users a single-interface experience to access CWA 2007 through Internet and Intranet.
Here is the rest of the story.