In Microsoft's terms, it's about unifying the communications experience and making it more intuitive, and that of course means using Microsoft software. The opportunity is huge: transform our idea of a phone. The market is huge: any office using Exchange.
The basic idea is to give users integrated audio and video communications, IM, and data sharing all in one interface to give a much better sense of who's available to speak and when, and to get the Microsoft name on that little screen on IP phones.
But as I mention in my story on the Microsoft announcements, it's a complex vision. Microsoft doesn't deal in hardware, and that's why it needs to leverage standards like SIP as well as all the partners it can get (to beat them to the punch). It'll also have to get over the stigmas of insecurity and unreliability (to compete with the likes of Cisco) and integrate the consumer into its vision (to compete with upstarts like Skype) if it wants to ultimately succeed.