In the July 2007 "Communicator 2007 - What's new Guide" the following was published:
Ability to plug in custom applications. Application developers and administrators can now plug in custom applications to the Communicator main UI and Conversation window entry points. For example, a developer could integrate an Interactive Voice Response (IVR) system application that sends a SIP message to Communicator to display a custom HTML page in the Communicator Conversation window.
Any idea of how custom applications can be plugged into Office Communicator?