Surely, Microsoft will use their new "Real-Time Communications Server 2003" at the heart of their office messaging infrastructure., see:
The RTC Server uses ietf standards like SIP for session control, SIMPLE for presence detection (am I sitting at the computer) and RTP for transmitting the data (most likely over UDP, potentially on top of TCP) and RTCP for controlling the flow.
What seems to be missing is the SOAP binding to RTP.
This seems a bit too obvious. Perhaps we will see an "enterprise" version that is shipped with the binding and resolves SIP addresses to WS-Addressing addresses. Don't know.... if anyone out there knows, shoot me a note.
MS has a good whitepaper on the technology.