A Microsoft spokesperson contacted me to say that "the stat Ballmer gave was actually a restatement of the same data already announced in December at the [Windows] Store event regarding the number of Windows 7 devices that could potentially upgrade to Windows 8 -- so there's no new data here. This was actually just a case of some confusion in terms of wording."
In other words, Microsoft says that the 500 million figure Ballmer gave doesn't refer to the number of copies of Windows 8 Microsoft expects to sell. Rather, it's the number of Windows 7 licenses sold, each of which could be upgraded to Windows 8.
Microsoft says that Ballmer was just restating what Ted Dworkin, Partner Program Manager for the Store, wrote in a blog post at the time of the Windows Store event:
"We've just passed the 500 million licenses sold mark for Windows 7, which represents half a billion PCs that could be upgraded to Windows 8 on the day it ships. That represents the single biggest platform opportunity available to developers."Not having been at the event covered by the AFP, the Seoul Digital Forum in South Korea, I can't vouch for what exactly Ballmer said. But in fairness, I thought you should hear Microsoft's side of the story.