Microsoft watcher Mary Jo Foley reports that while Windows 8.1 is expected to hit release to manufacturing (RTM) phase by the end of August, it will likely take until mid-October before the update will see its public release.
This will give Microsoft more time to fix bugs and allow for a "bigger bang" launch event.
Unlike the case with Windows 8, however, I'm hearing scuttlebutt that Microsoft is not planning to make available the final Windows 8.1 bits to its MSDN or TechNet subscribers shortly after the release RTMs. In the case of Windows 8, Microsoft RTM'd on August 1 and made the RTM bits available to MSDN and TechNet subscribers in mid-August, even though consumers couldn't get the final version until late October 2012.
The new word, one of my best tipsters tells me, Microsoft is going to hold off on making available the final Windows 8.1 bits until mid-October 2013 or so. That will be both the general availability date, as well as the "launch" date when new hardware running those bits will be available.