Given the history of the vnode interface I find it curious why the chief advocate of vnode interposition (judging by the number of papers on the subject), Sun Microsystems, has not included any fundamentally new vnode interface in their operating systems. Sun has released over half a dozen new versions of their Solaris operating system in the past few years, so they certainly had the opportunity to include a new interface had they wanted to.
I've had several personal communications with experts in the field: Brent Callaghan, Glenn Skinner6 and a few others who chose to remain anonymous. Unanimously, they told me that while they thought that vnode interposition is desirable, more pressing projects were given higher priority. They cited management concerns over the commerciability of stackable file systems, the overall cost of making such radical changes to the operating system, and the perceived lack of short-term benefit from making such changes. In addition, management did not want to incorporate any changes that degraded performance even slightly in the then-fledgling Solaris 2.x operating system.