IDC that VMware's share of new x86 virtualization software shipments was 44% in the second quarter, with Microsoft clocking in at 23%. VMware's lead in last year's second quarter was 51% to 20%, and its lead in the first quarter of 2008 was 42% to 18%, according to IDC.

IDC attributed Microsoft's growth to the general availability of , a notion VMware officials scoffed at. Hyper-V was on June 26, with only a couple business days left in the second quarter, Mike DiPetrillo, a principal systems engineer at VMware, wrote in a that analyzes the IDC report.

"So did Hyper-V really ship enough units in 2 days to get 23% market share? I doubt it," DiPetrillo writes.

DiPetrillo accused IDC of basing the 23% figure only on "unit shipments from the OEMs," but this does not appear to be true. The IDC survey did examine OEM vendors like HP and Dell who offer servers that have already been virtualized, but that was a separate comparison and there's no indication in IDC's report that the VMware/Microsoft market share comparison was limited to OEM sales.

IDC analyst Brett Waldman explained IDC's methodology in a phone interview Wednesday, saying the analyst firm determined market share by surveying more than 2,500 virtualization users from 35 countries, examining public filings and having conversations with vendors. IDC's user survey looked at all new server virtualization licenses, regardless of whether they were sold through OEMs or other sources, he said.