It appears Microsoft has signed off the final version of Windows 8.1 at build 9600. The build was compiled yesterday at 9:40PM in the winblue_rtm branch. The rumor comes from a source who has been behind multiple information leaks in the past. Rumors yesterday claimed that Microsoft were going to compile the Windows 8.1 RTM at build 9600, and today's information seems to confirm that.
The full build string reads 9600.16384.winblue_rtm.130822-2140, which was compiled on August 22nd for testing. It was said that if 9600.16384 had a problem with it, the 16384 part of the build string would have changed. It has not. So can we expect an official announcement from Microsoft soon?
What's more, it appears Microsoft is ready to send the final code off to OEM's immediately. According to another post on the same thread, Microsoft is preparing to send the final bits off to OEM's within the next few hours for pre-loading, testing and other goodness.
Of course, until this information is concrete, we ask you to read with a grain of salt. If it were to be true, we can expect Microsoft to make an official announcement soon.