I did some additional research on this and found the following "very brief" comment by Microsoft. I imagine there comment "some innovative features" is really equivalent to proprietary extensions that will not be part of the SIMPLE protocol? Microsoft "innovating" again :)

Q. Can clients developed by companies other than Microsoft interoperate with Live Communications Server 2003?

A. In general, interoperability is limited to the Microsoft Windows® Messenger 5.0 client or applications built on top of V1.2 RTC Client application programming interfaces (APIs). Microsoft has developed some innovative features in the real-time communications area. Several extensions made by Microsoft that use the SIMPLE protocol are not yet implemented by other companies.