The old cliche about 'lies, damned lies and statistics' applies perfectly to the numbers being bandied about by the three main application virtualization vendors: Citrix, Microsoft and VMware.

Microsoft said last month that it had sold 6.5 million licenses of its Microsoft Desktop Optimization Pack (MDOP), its bundle of desktop and application virtualization software available only to enterprise customers subscribing to its Software Assurance program.

That more than doubled the number of Microsoft's desktop and application virtualization customers had just four months earlier.

But MDOP buyers could easily have bought the bundle to install other components, and left Application Virtualization (formerly known as SoftGrid) untouched, pointed out Brian Madden, an independent virtualization technology analyst and blogger.

"Saying you've sold 6.5 million MDOP licenses is like claiming 40% of consumers are using Vista today," he said.

By contrast, VMware's competing ThinApp application had slightly more than 1 million users when VMware bought its maker, Thinstall, in January, according to Ed Albanese, senior product manager at VMware. He declined to provide an updated figure on the number of users of ThinApp, for which VMware announced a major new release Monday.

Madden says Microsoft's Application Virtualization is "undoubtedly more popular than ThinApp. But is it 6.5 to 1? That's probably a stretch. It's probably more like 4 to 1."

If you think Redmond is exaggerating, take a look at the numbers Citrix has bandied about in relation to its equivalent product, Xen Desktop. Citrix bought XenSource in August. Since then, it has claimed that its XenApp application virtualization software has 70 million users - or nearly 11 times Microsoft's count.

The problem, says Madden, is that XenApp is really a new name for Citrix's long-standing Presentation Server software, which can either enable old-fashioned Terminal Services-type application delivery, or new-fangled application virtualization/streaming.

"They've taken their legacy server application and thrown in the capability to stream applications virtually," he said. "So technically, there are 70 million users authorized to use XenApp, but only a couple of million are probably actually using it."

While all three products do basically the same thing, each vendor has slightly different goals. Citrix wants enterprises to manage all of their local applications through XenApp, whether they be those delivered via terminal services or through application streaming, Madden said.

Microsoft wants enterprises to replace the conventional Windows installer for installing client-side software with Application Virtualization, which it hopes will also "push Vista adoption," according to Madden.

VMware, meanwhile, is pushing enterprises to adopt its Virtual Desktop Infrastructure (VDI) technology. Enterprises would create one master image that all desktop users would connect to, he said. Thinstall would provide the customization piece, by enabling each client PC to download the applications its user requires.