The subscriptions essentially are the same as a sponsorship, he argued. But DiPetrillo acknowledged he doesn't know for a fact that Microsoft subscribed.

In his blog, DiPetrillo also claims that IDC failed to count usage of VMware Server, a freely downloadable product. This appears to be untrue, as IDC specifically stated that VMware's market share includes both the and VMware Server. DiPetrillo said "I'll be happy to post corrections on the blog if there are corrections to be made."

DiPetrillo's blog states that it contains his personal opinions, and is not hosted by VMware or vetted by anyone at the company.

VMware's internal public relations team urged reporters to read his blog post, however, and said VMware disagrees with IDC's findings.

"Our own data leads us to believe that VMware's unit market share is much higher than the IDC numbers, and that competitor market share is much lower," a VMware spokeswoman wrote in an e-mail to Network World.