The Windows 8.1 release date is October 17 - it will be available to existing Windows 8 users for free on Windows Update. It will be available to buy in retail from October 18.
The official confirmation from Microsoft: "Today we are excited to share that starting at 12am on October 18 in New Zealand (that's 12:00pm in the UK and 4:00am in Redmond - October 17th). Windows 8.1 will begin rolling out worldwide as a free update for consumers on Windows 8 through the Windows Store. Windows 8.1 will also be available at retail and on new devices starting on October 18 by market."
October will mark a year since Windows 8 was released. There's also aWindows 8.1 RT version, too, if that floats your boat.
To whet our appetites in the meantime, Microsoft has released a preview version, called the Windows 8.1 Release Preview. It's now available to download.
The big headline news is that the Start button has returned to Windows 8 with Windows 8.1, although it still goes to the Start screen rather than the Start menu. There is also more integration between the desktop and Start screen to stop the jarring of the two interfaces.
On stage at the Microsoft Build developer conference in San Francisco, Steve Ballmer said that in coffee terms, Microsoft was "refining the blend" between the desktop and Modern UI interfaces and a lot of Windows 8.1 enhancements have been designed to make the change between the two interfaces far less jarring.

Steve Ballmer

Ballmer also promised plenty of new, smaller Windows 8 tablets would be released as well as new apps from Flipboard, Facebook and the NFL - though we didn't see any screenshots of these in action.
In our own hands on review, Mary Branscombe sums the new release up as a moderate success. "Windows 8.1 isn't a whole new operating system: it isn't the same leap as Windows 7 to 8. But it's more than a service pack as well.
"Performance feels generally faster, even for simple things like zipping up files. The interface changes won't please everyone, especially if you liked the Windows 8 Start screen and don't feel you need for yet another Windows key on screen - or if you were hoping for the Windows 7 Start menu back.
"Some things, like customising tiles on the Start screen, feel a little more long-winded until you get used to them. But generally the interface feels more consistent and easier to learn. And the expanded PC Settings gives the mass of control panel options a clean, simple interface that Windows has needed for years."