Office Communication Server 2007, Communicator 2007 and Live Meeting is soon to be released, so it is not curious that developers are interested to use it as a new and exciting development platform!
One of the most complete references about Unified Mmessaging (UM) that is aimed for developers is Chris Mayo's Blog - VOIP Development. He is describing each SDK that will help developers to make new collaborative and multimedia applications:
Office Communicator 2007 SDK: This SDK provides the ability to integrate presence into your applications and automate OC 2007 in order to launch voice, video or IM communications (including multiparty).
Unified Communications AJAX SDK: This SDK allows you to build presence and IM communications into your web apps using AJAX.
Unified Communications Managed API SDK: This is a library of managed classes that allow you to create server side or middle tier applications for SIP processing (incoming, outgoing or both). Use this API to send out broadcast IM, or use the API to register the presence of an IM Bot that works like a contact for users in OC 2007 and allows users to IM the Bot queries for quick access to information.
Speech Server 2007 Extensions for Visual Studio 2005: This API includes the SDK and designer for creating TTS/ASR applications using Speech Server 2007. Use this SDK to create applications that allow users to phone your application to get information or use the SDK to call users to drive the next step in a workflow.
Office Communications Server 2007 SDK: This API allows you to customize the SIP processing of OCS 2007 to add custom business logic. Examples include adding a custom disclaimer to IM conversations for legal or compliance reasons.
Unified Communications Client SDK: This SDK allows you to create your own SIP clients, such as a specialized IM application for a specific industry (finance, health care, etc.).