Microsoft has now released the Windows 8.1 ISO files that can be downloaded and used to install the Windows 8.1 Preview Release. Prior to the ISO files the only way to retrieve the preview software was through Microsoft's standard Windows Store mechanism. The ISO files are for more advanced users who need additional options for deploying the updated operating system on another PC, on a split partition, or even to run in a Virtual Machine.
Available in multiple languages, the downloads are located here: Windows 8.1 Preview ISO files
It should be noted that the ISO files are NOT for Windows RT. Windows RT still requires that Windows 8.1 Preview be installed from the Windows Store.