The free (120 day) evaluation version of VMM is now live on the TechNet TechCenter. You can also try the eval version as a VHD and run it on a VM!
Download the 120-day trial software
System Center Virtual Machine Manager (VMM) 2007 provides a comprehensive management solution for the virtualized data center that enables increased physical server utilization and centralized management of virtual machine infrastructure.
Download the VHD
The preconfigured files come with all the necessary components to get VMM up and running quickly as a virtual machine. Once the VHD is downloaded and joined to a domain, you can begin testing all the functions of VMM including the Self Service Administration Portal and Intelligent Placement capabilities.
When it is time to move to the full version, all you have to do is uninstall the eval version using the "Retain data" option, then install the full version.