Microsoft is doing a 180 degree turn on a policy regarding virtual machines, most likely to appease their customers. Formerly, they had forbid the installation of Vista Home Basic and Vista Home Premium on virtual machines - regardless of what virtualization suite you were using, even their own. Now they have relaxed that rule, and will provide legal licensing options for doing such.
This might ease restraints in some development environments, and the article mentions Mac users whom may want the capacity to use Vista in a VM without having to buy an expensive license for the premium editions. The actual reasons behind the move aren't cited by Microsoft, but it may be an attempt to encourage more Vista adoption. Particularly for those who choose to go the legal route and buy licenses for their VMs, this will be helpful.