Karras is skeptical of using open-source software in a medical environment. "The problem is that there's nobody to call" if a problem arises, he said. "And if there is somebody to call, it means it's not really open-source and you'll be paying for support."

Also skeptical is Mike Simson, director of operations at American Modern Insurance Group Inc. in Cincinnati, which has 170 virtual servers running on 140 server boxes. "My philosophy is the same about open-source as what they used to say -- that nobody ever got fired by buying IBM," Simson said. He also pointed to cost reductions in server maintenance using virtualization but couldn't provide an amount.

Server virtualization is also being used in smaller companies. Riester, an advertising firm with 100 employees in Phoenix, has been using a single server box with three virtual servers running on VMware, said Dan Peterson, director of IT. He said that the cost of VMware has pushed him to consider XenSource, which he is testing currently. VMware charges for management software, which the company is trying to avoid, and Peterson said he doesn't have any reservations about trying open-source in his shop.

IDC has seen enormous growth in virtualized servers, noting nearly 500,000 virtualized servers should ship in 2006, up from nearly zero just three years ago. By 2009, the number shipped should be 1.2 billion, according to the research firm. IDC analyst Michelle Bailey said a survey of IT managers in the U.S. by IDC last year showed widespread adoption of the technology, with even more growth for a variety of applications showing up in a more recent survey not yet released. "I'm shocked by the level of adoption of virtualization," she told a conference audience.

VMware dominates the commercial server virtualization market, competing with Microsoft Corp., SWsoft Inc. in Washington and Cassatt Corp. in San Jose, Bailey said. She said that with more virtualization being deployed, the next trend to follow will be how management software vendors provide tools to manage virtual environments. Last week, for example, Opsware Inc. in San Jose announced a strategy for virtualization management.