In hopes of pulling some attention away from Vmware and other vendors, Microsoft is beefing up their virtualization offerings. Volume Shadow Services and clustering are the two big additions here, both targeting larger companies employing Windows Server 2003 Enterprise or Datacenter edition.
The move is just another iteration towards their ultimate goal of Viridian, though it is an important one. Other improvements include guest OS support for Solaris 10 and Novell SLES 10. Viridian won't be available for some time yet, and Vmware already has a huge advantage over Microsoft in terms of OS support and adoption.
Anyone who wishes can of course download the update for free.