Volume-licensing customers now can get the Windows 8.1 Update and Windows Server 2012 R2 Update from the Microsoft Volume-Licensing Service Center (VLSC).
Microsoft made the latest updates available to those users on April 14, as planned. In addition to those two updates, Microsoft also made the Update available to those volume licensees running Windows Embedded 8.1 Industry.
As Microsoft officials note in the Springboard Series Blog post announcing volume availability, the VLSC and MSDN updated media doesn't include the full set of fixes that those downloading the new Updates got via Windows Update. (MSDN/TechNet subscribers got the Updates on April 2; non-volume/non-MSDN Windows 8.1 users got the updates via Windows Update on April 8.)
As a result, "Windows Update and WSUS will offer the Update again to devices that are deployed by using this (MSDN and VLSC) media. This is expected and does not result in the full reinstallation of the Update but only a small component of it. The rest of the update will not be downloaded or reinstalled."
The Springboard post also acknowledges a problem discovered last week that affects a subset of Windows Server Update Services (WSUS) customers attempting to apply the Update to their machines. Those with WSUS 3.2 running on Windows Server 2003 SP2, Windows Server 2003 R2 SP2, Windows Server 2008 SP2 or Windows Server 2008 R2 SP1 are among those that may encounter problems. Microsoft is advising those affected by the WSUS glitch to read KB2959977 for guidance.
It's worth pointing out again that this Update for Windows Server 2012 R2 and Windows 8.1 is a requirement for obtaining future patches, fixes and updates. Microsoft will no longer update those Windows 8.1 and Windows Server 2012 R2 users who don't apply these updates starting with the next Patch Tuesday, which is May 13.
Windows 8.1 Update also can be installed on smaller and cheaper Windows 8 logo-certified devices with smaller disks in a way that leaves more room for apps and data. That new deployment option, called Windows Image Boot (or WIMBoot) is supported with all SKUs of Windows 8.1 Update. Here's more information on how WIMBoot works.
Update (April 16): Microsoft has fixed the WSUS issues and has extended the deployment deadline for the Updates from May to August for business users. More information here.