This subject seems to pop up frequently in my client engagements. Standards like DocBook and DITA both have review and annotation markup. Yet, many structured authoring tools use their own markup for reviewing and annotating XML draft content. Some use processing instructions; others use special namespaced elements. None, to my knowledge, recognize the specific elements as annotations.
For many of our clients, a clearly defined review process is critical to the overall lifecycle of the content. If vendors can't support specific each standards-specific annotation markup, it makes me think that a common markup language for reviewing and annotation would be extremely useful.