All About Microsoft posted a story that Microsoft is on track to deliver Windows 8.1 by August 2013 to its PC/device partners

Microsoft will make Windows 8.1 available to all Windows 8 and Windows RT users as a free upgrade through the Windows Store.

While I've heard some Microsoft watchers predicting that RTM and general availability of the Windows 8.1 bits won't happen simultaneously, this is not what I have heard from my sources. While new devices preloaded with the Windows 8.1 bits will probably not be out until late September/early October, I have heard that the general availability of the Windows 8.1 bits won't be held back this time around. I think users will be able to get the 8.1 bits simultaneously or very shortly after RTM.

  Microsoft confirms Windows 8.1 to RTM by August