Ken Oestreich, of Cassatt, posted his own review of Paul Maritz's vision of a virtual datacenter operating system. It can be found here. As one might expect, it points out that others, including Cassatt, Virtual Iron and others have been implementing versions of this concept for quite a while.
An important point covered in his analysis is that both physical and virtual systems must be supported by any viable datacenter virtualization technology. This technology must deal with all of the important operating systems running on industry standard system. It must also deal with the fact that datacenters look like computer museums and all of the technology, not just today's industry standard systems, must be addressed.
It's worth a read.