Projects like the VMware Player, a stand-alone tool for hosting a VMware-created VM on any Windows desktop system, seek to position the VMware file image as a de facto standard for delivering appliancelike application functionality. Already, a large selection of prebuilt VM images is available through the VMware Web site, most containing open source OSes and applications that can be freely redistributed.

Microsoft, by contrast, has allowed its offerings to languish. Virtual PC, once a strong competitor to VMware when it was still a Connectix product, has only recently been updated. Virtual PC 2007 adds support for Windows Vista as a host operating system but not much else. It still doesn't support 64-bit computing and continues to lag behind VMware Workstation in areas like USB device integration.

One wild card in the VM equation is Citrix Systems. Long the dominant player in server-based computing, Citrix now portrays itself as the true pioneer of application virtualization. Cut through the hype, however, and you'll find an amalgam of repositioned products punctuated by the addition of an application virtualization and streaming solution similar to SoftGrid. The success of the Citrix strategy will hinge on how well it can integrate this functionality, known as Project Tarpon, with the myriad protocols and presentation layers that make up the Citrix stack. Project Tarpon becomes part of Presentation Server in March.

Interestingly, VMware could learn a thing or two from the Citrix experience. Many of the same pressure points that held back server-based computing -- poor local hardware support, limited client mobility, massive back-end hardware requirements -- are present, and in some cases exacerbated, in Virtual Desktop Initiative deployments. Instead of hosting multiple user sessions on a single Terminal Server image, you're now hosting the equivalent of multiple Terminal Servers, each with a single connected RDP (Remote Desktop Protocol) user. The scalability implications are frightening: easily 10 times the hardware required to support an equivalent server-based computing load.

Just as Citrix has reinvented itself as a virtualization trailblazer, VDI players such as Wyse and Neoware and protocols such as RDP and ICA (Independent Computing Architecture) are looking for a second life. They may find, however, that the grass is no greener on the VDI side of the fence.