From his blog Compl Torris, TechNet Ireland Manager, unofficially announces Microsoft will offer its customers purchasing Windows Server 2003 Datacenter Edition with Volume License capability to run unlimited virtual machines hosting any flavor of Windows Server, Enteprise or Datacenter editions. With effect since 1st October 2006.
In the official announcement Microsoft also stated Windows Vista Enteprise Edition customers will be able to run up to 4 virtual machines for free (Ben Armstrong specifies this policy works with any virtualization product, not just with Virtual PC).