Avaya have just published a status report (PDF) on Unified Messenger, the product I work on, and its installation at Microsoft. It gives an idea of what is involved in a large-scale unified messaging implementation.
I've been responsible for the application server framework of the product for several years now. Major projects have been designing and implementing support for COM service objects (making extensive use of the Universal Delegator for pre and post-processing COM calls) and now .NET objects. The various components in the system communicate using three remoting technologies: a now deprecated proprietary remoting infrastructure which uses named pipes, DCOM, and most recently, .NET Remoting. Interestingly for the latter we have returned to Named Pipes, using a Named Pipe custom remoting channel to achieve adequate performance and built-in security.