http://www.microsoft.com/mscorp/execmail/

Bill calls out BizTalk Server and Host Integration Server's key role in interop pretty clearly in this paragraph:

"Some of Microsoft’s server products are focused squarely on interoperability, such as Host Integration Server for mainframe connectivity, BizTalk Server for heterogeneous integration across multiple applications, or Identity Integration Server, which helps simplify user authentication and management across diverse systems. These resulted from many years of working to understand customer needs and their existing environments."

Good stuff.