VMware's new ESXi 5 hypervisor can run on Apple's discontinued Xserve hardware, and can virtualize Mac OS X Server 10.6.x as a guest OS. ESXi runs on the bare hardware, without a host OS. Parallels offers a Parallels Server for Mac Bare Metal Edition that is similar in concept. ESXi 5, however, is part of VMWare's vSphere 5 system, which enables enterprise organizations to virtualize IT services and host them in their own cloud for private use, or to offer services to other organizations. vSphere runs in data centers using ESXi hypervisor to support multiple virtual machines across multiple hardware servers.
The announcement, however, dashes the hopes of those who thought vSphere 5 might be a replacement for the Xserve, after VMware noted last spring that the new version would support the hosting of Snow Leopard Server. The rumor was that Apple would allow VMware to support an older version of Mac OS X Server on non-Mac hardware as part of a vSphere installation. Some even speculated that vSphere would be Apple's official strategy for enterprise customers to replace Xserve.
With vSphere 5 and ESXi, however, VMware is clear that this isn't the case, stating, "To run Mac OS X in a virtual machine, ESXi must be running on Apple branded hardware."