Although many open-source vendors continue to make products that work best on Linux, some are also questioning a decision to ignore the huge Windows market.

"As an open-source vendor, we believe in choice," said Ram Venkataraman, director of product management for JBoss. Half of JBoss' customers run Windows. And despite JBoss' acquisition by Linux vendor Red Hat earlier this year, Venkataraman said the company has no plans to cut out its Windows users.

"It's important for Java deployments to run on Windows," he said. "If you look at Web services, it's all about interoperability."

The need to interoperate and cut costs led Sherwin Lu, director of application infrastructure for Chicago-based preschool chain Le Petite Academy, to upgrade to the JBoss application server on top of Windows Server 2003.

"It felt a little risky" to move to J2EE from a Visual Basic 6 environment, Lu said. But the cost of training his staff was about the same as it would have been if he had upgraded to a .Net infrastructure. Moreover, by adopting JBoss over other proprietary application servers, Lu figures he saved about a million dollars in license fees alone. And by staying on Windows, he avoided the pain and cost of "rehiring my entire sysadmin and support team."

Even Web servers -- a longtime sweet spot for the LAMP stack -- are increasingly being run on Windows.

According to Mark Brewer, CEO of US-based Covalent Technologies, almost a third of the customers it supports on the Apache Tomcat application server are running it on Windows.

"That had been 15 percent to 20 percent historically," Brewer said. Almost a fifth of Covalent's customers also run the Apache HTTP Web server on Windows, which Brewer considers equally significant, considering that Microsoft bundles a competing product, Internet Information Server, with Windows Server. More than ideology, that factor -- that Microsoft makes a huge number of business applications, a number that is only increasing -- could eventually limit the growth of open source on Windows.

"If I've already got Microsoft installed in the box, why would I bother to throw it away and install something else?" said Mike Olson, vice president of embedded technologies for Oracle and former CEO of Sleepycat Software. Sleepycat, before it was acquired by Oracle in February, made an open-source embedded database that competed with Microsoft. "It's a friction-y thing to have to do," Olson said. "So as long as it doesn't suck, you're going to stick with what you've got."