Microsoft announced the availability of Windows 8.1 RTM version for the MSDN and TechNet subscribers today. That means, if you have any one of the MSDN or TechNet subscription, you will be able to download it from your subscription page.
General availability of Windows 8.1 will be on 18th October 2013. If you hold a Windows 8 installation, you will be able to upgrade to Windows 8.1 from Store after that.
Earlier to that, we already heard that, there will not be any availability of Windows 8.1 for MSDN subscribers prior to 18th October. But there were lots of discussions on that front from the developers and/or partners who were building apps for Windows 8.1 and finally Microsoft heard that and released the RTM version for the developers, partners through the MSDN/TechNet subscription benefit.
If you are a developer and holding the above subscription, you can now go and download Windows 8.1 and Windows 8.1 Pro from your subscription page. Windows 8.1 RTM Enterprise edition will be available through the same subscription page later this month.
Microsoft also released Visual Studio 2013 RC today, which you will need to build Windows Store applications targeting Windows 8.1. Keep in mind that, Visual Studio 2013 RC does not support Windows 8.1 preview.