Desktop virtualization has its dominant vendors, namely VMware and Citrix Systems. But in an embryonic field, consider the alternatives. I'd like to cite Neocleus, an Israeli firm, which is focused on running the virtual machine at the desktop, not on a central server, under a desktop hypervisor.

Desktop virtualization from the major vendors arrives with the ability to scale to thousands of users. That's because they realize a key piece of access management needs to be built into the process, a connection broker that takes an incoming request, identifies the requestor through Active Directory, and then makes the connection to the provisioning server. They can handle hundreds or thousands of users seeking to activate their virtual machines at the same time.

But what if you only want to virtualize a handful of users at a time? Or what if you want to virtualize hundreds of users, but those users are scattered in small pockets around the company? In that case, you don't want a connection broker so much as a distributed method of provisioning the occasional end user.

When it comes to desktop virtualization, it's not yet clear what constitutes the best division of responsibility between central server and virtualized end user.

Consider Neocleus. Last week I talked to CEO Ariel Gorfung, and he emphasized the advantage of distributed execution of virtual machines on the user's existing hardware. The Xen-based, end user Neocleus VM is generated on a central server but runs on the user's machine. Because the user's environment becomes a virtual machine, it can be encrypted and locked down.

This sounds like Phoenix Technologies' HyperCore, also based on Xen, or VMware's ACE, says Rachel Chalmers, virtualization analyst at The 451 Group, and she's right. The virtual machine is running on the user's hardware, and it's got the security barriers that virtual machines can provide, but ACEs are still tied to a central ACE Management Server.

Gorfung claims Neocleus has modified Xen so that it is a client hypervisor. Hypervisors thus far have resided on servers, hosting one or more virtual machines above them and dispensing with the host server's operating system.

Gorfung says Neocleus is less like a user's virtual machine, still tethered to a central server and more like a user hypervisor, running "side by side" with the virtualized operating system and applications. The user gets the standard desktop but also has the option of "creating an extra partition in which you watch DVDs without booting all of Windows," says Chalmers.

Gorfung says Neocleus is trying to create a desktop hypervisor framework as open source code, and any vendor should be able to create a software appliance -- an application and operating system that's been combined into a virtualized file set -- and plug it into the framework. Under such a scheme, the whole operating system debate between Mac OSX, Windows, and Linux goes away. Each is reduced to a virtual machine running under a desktop hypervisor and a broader world of applications opens up to PC users.

It's a great vision, but we're not there yet. It remains for Neocleus or Neocleus partners to demonstrate the power of a type 1, desktop hypervisor -- that is, one that acts as a direct, intermediary between hardware and virtual machine, not an emulation of the hardware in software above the bare metal.

Both user security and application performance could benefit from this approach. Addressing scalability, the way Citrix and VMware already have, can come later.