Citrix Systems just announced the availability of XenServer 6

Improvements in XenServer 6 make it even easier to use, thereby accommodating customers who desire a cost-effective, powerful, and integrated server virtualization solution. Some improvements include:

• The much anticipated Open vSwitch (a flexible network utility that provides network management
  functionality within cloud infrastructures)

• Substantial optimizations for desktop virtualization

• Full support and integration with Microsoft System Center with the ability to manage virtual machines using Virtual Machine Manager

The core of XenServer 6 also builds upon the recently upgraded Xen 4.1 hypervisor – the latest release by the open source community.