Steve Ballmer has announced the beta release of Microsoft Virtual Server 2005 Service Pack 1 at the Microsoft Management Summit (MMS) 2005.
"The Virtual Server 2005 Service Pack 1 (SP1) beta version is available today, delivering 64-bit compatibility and improved performance and availability. Virtual Server 2005 SP1 will enable support for Windows Server (TM) 2003 x64 editions as host operating systems and, along with performance improvements in the virtual machine software, will enable customers to further increase the efficiency of their hardware environments. Virtual Server 2005 SP1 beta is available at https://beta.microsoft.com with the guest id "vssp1BetaTester". The final version is scheduled to be available by the end of the year."
Following that public commitment beta invites have been sent out to potential testers.