VMware commands huge market share, at about 85 percent, while the other players are in single digits. That's likely a primary reason for the partnership. To compete with VMware, or at least substantially close the gap, it will take the joint efforts of companies such as Microsoft and Citrix. It helps that Hyper-V will be bundled with Windows Server 2008, being released next month. The chief complaint about VMware is the high cost of its products.