In the "thinking out loud" category: Microsoft's Windows RSS Platform is an innovative approach to the problem of XML syndication. It provides a common integration point with differing levels of integration. It becomes a policy enforcement point for security, network management and so on. It helps provide a generalized platform mechanism for all applications to ensure a consistent user experience. It's not a perfect play by Microsoft (the Outlook team for instance seems to be building a somewhat redundant capability and the platform is essentially a V1 technology), but it will help establish generalized client infrastructure for XML syndication.
So why not duplicate this approach for presence? Let's call it the "Windows Presence Platform". Such a platform would provide a common set of client interfaces that multiple "watchers" could report into, with varying levels of integration from basic presence awareness to more complex presence based on a variety of activities and situations. It would also allow for non-SIP based applications to interact in a consistent fasion without having to know the nuances of the presence system itself (in fact, a Windows Presence Platform might support a variety of protocol integration points. It would not require Office Communicator to be on the machine at all so other types of client-front ends could virally take off.
Again, just food for thought without a lot of technical foundation, but I think the model could work (a headless presence client aggregator).