The rumour has circulated for a while, but it looks like Windows 9 will be available free to owners of Windows 8. That's according to Indonesian news site Detik.com, which has published a statement by Microsoft Indonesia's Andreas Diantoro seemingly confirming the news. According to a Google translation of the article, Diantoro said Windows 8 will "update automatically" to the new edition of the operating system when it releases.
According to the article Diantoro makes this claim regarding codename 'Threshold', which is believed to be Windows 9. Whether the upgrade will apply to owners of Windows 7 or Vista is not specified.
The news comes ahead of an expected preview edition of Windows 9, which will reportedly release either September or October. Given we're fairly late in September now, it's reasonable to assume the date has been pushed to October, though Microsoft is expected to officially unveil the new OS during a September 30 event in San Francisco (that's tomorrow). Chief among the expected changes is the glorious return of the Windows start button.