This is not the first public release of Windows 8 - The company released a developer preview to give developers an idea of what to expect in Windows 8. The Preview release which was a pre-beta release demonstrated the tile based touch friendly Metro UI, the new notification system among other features which will make their way to the latest version of Windows.
Giving developers a chance to work with the new UI Microsoft will open the Windows App Store for developers to Submit their apps along with the beta release of Windows 8.
Tech experts and analysts expected Microsoft to unveil Windows 8 during the CES event in January as CES 2009 hosted the beta release of Windows 7. But Microsoft want to ensure that they have a great product ready before opening it up to the public.
Microsoft hasn't revealed that the visual changes we can expect with Windows 8, but we figure Microsoft will continue to improve the experience they offered with the Developer Preview.
source - Microsoft