Alessandro's blog quotes Mike Neil (MN), Virtual Machine Technologies Product Unit Manager at Microsoft, RE the Active Directory requirement for Virtual Machine Manager Beta 1:
VI: There are any plans to provide a VMM version not requiring Active Directory?
MN: System Center Virtual Machine Manager beta 1 will require Active Directory and the initial thinking is for the final released product to be the same. However, the primary reason for getting the software into customer hands in beta form is to identify opportunities to improve the software. Active Directory is currently required for security purposes to ensure that access to images on the network is authenticated and secure. Active Directory also provides benefits by allowing the SC VMM administrator to identify and understand all the physical servers running virtual machines that are installed in their environment and the use of Distributed File System to replicate images automatically across the network.
This is almost word for word the same answer you can find in the Virtual Machine Manager FAQ.
If you missed Eric Winner's WinHEC demo during BobMu's speech fast forward to about 38 minutes into the playback.
You may also want to check out:
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Virtualization Technical Resources (MOM Pack, VSMT, Host Clustering Whitepapers, etc)
Download Virtual Server 2005 R2 Enterprise Edition (free, full version, not time-bombed)
Download Virtual Server 2005 R2 SP1 BETA (includes support for VT systems)
Download Virtual Machine Additions for Linux