Om Malik had a thought-provoking article on Business 2.0 about Microsoft's VoIP plans.  They are nothing less than ambitious: become the key enabler of the technology (via Longhorn - the next-gen OS that will include SIP addresses everywhere, a corporate IM client codenamed Istanbul, and its newest release of LCS).  Again, I personally believe that Microsoft will not quite compete against the likes of Avaya, Nortel, Cisco and other PBX/IP PBX vendors. 

However, there is little doubt that the folks from Redmond want to be an integral part of applications that will be key in providing the productivity enhancements that VoIP promises to deliver.  For instance, the tight integration of Istanbul and LCS can mean that a conference call can be simply initiatied by dragging and dropping (via MS OLE) a bunch of contacts from a Microsoft Outlook address book.  A phone number in that same contact list might have an embedded SIP link that will automatically spawn a VoIP call.  This means Microsoft will be, at least on the application side of things, the de-facto platform for VoIP in the enterprise (despite the fact that the currently, the only two vendors running Microsoft Windows OS on their IP PBX servers are Cisco, with the Call Manager, and Siemens, with the HiPath 5000).