Citrix is widely acknowledged as a major player in the desktop virtualization market, but faces an interesting competitive challenge in that many or most Citrix XenDesktop customers are using technology from rival VMware on the back end.

Recently, Burton Group analyst most Burton Group clients that have deployed Citrix's virtual desktop infrastructure are still using VMware ESX servers to host the desktops. That's primarily because VMware offers a memory overcommit feature that allows greater density and consolidation levels, he said.

Citrix's John Humphreys, senior director of marketing for virtualization and management, acknowledged this problem in an interview Wednesday, but said the trend is shifting.

"A year ago, about 90% of our sales of XenDesktop were hosted on VMware [servers]," Humphreys said. "Through the second or third quarter, that percentage is down around 60% to 70%."