VMware said it was releasing the software ahead of schedule, although it appears to be in line with its plans for a second quarter release. "I think they had June in mind so they're pushing out a little earlier than they expected," said Burton Group analyst Chris Wolf.

BayScribe, an Edgewater, Maryland, company that provides a dictation and transcription service for doctors, has been beta testing vSphere and will move it into production as soon as it can, said Steve Bonney, BayScribe's vice president of business development.

"By far the main reason we beta tested vSphere is for fault tolerance," he said. If its dictation system crashes because of a hardware failure, the fault tolerance allows doctors to keep dictating their notes without interruption, he said.

BayScribe has been using VMware's vMotion for its back-up, which creates a lag time when data is transferred between systems and requires doctors to hang up and dial in again if a system fails, he said.

Analysts say vSphere is important for VMware because it will help it maintain its technology lead over Citrix Systems and, especially, Microsoft, which recently entered the virtualization market with Hyper-V.