Appearing at the Seoul Digital Forum in South Korea this week, Microsoft CEO Steve Ballmer said that Windows 8 represented a “rebirth” of his company’s core OS that would trigger sales of 500 million units between its release later this year and the end of 2013.

“Windows 8 is certainly the most important piece of work we've done," he said. “It's really, in some senses, a dawning of the rebirth of Windows.”

That’s a bold prediction. Ballmer had earlier mentioned that he expected Microsoft to ship 350 million copies in calendar year 2012. If we assume that Windows 8 is available for the final quarter of 2012 plus all of 2013, a similar run rate would provide about 440 million units sold. Ballmer is claiming, however, that Windows 8 will sell well above that figure.

Having publicly stated such a number, Ballmer now faces a major embarrassment should Microsoft sell anything less than 500 million units in its first 15 months on the market.

To avoid such a tragedy, Microsoft is targeting Windows 8 almost exclusively toward consumers—not the business users who adopted Windows 7 in record numbers. This consumer focus includes pushing a special version of Windows 8 called Windows RT that will run on iPad-like, ARM-based tablet devices.

Microsoft is also making a related and major push into cloud computing with a consumer-oriented service called SkyDrive that is taking on new prominence and capabilities with the release of Windows 8. Ballmer says this focus will help drive adoption, too. “The number of cloud platforms around which software developers will do their innovation is not ever-broadening,” he said. “It’s really a quite smaller and focused number: Windows, various forms of Linux, the Apple ecosystem. [But] there will be few ecosystems that really can get critical mass.”

Consumers and business users alike will get a nearly feature-complete peek at Windows 8 in the first week of June. (My sources now tell me that the Release Candidate version of Windows 8 will likely ship on June 1.) The final release is expected by October 2012.