Windows 8.1 update just announced at the BUILD conference today is on MSDN/TechNet now, on Windows update on April 8th. In the read me file from MSDN is states
Windows 8.1, Windows Server 2012 R2, Windows RT 8.1 Update
Recommended Install Order
A servicing stack update is available for Windows RT 8.1, Windows 8.1, and Windows Server 2012 R2: March 2014:
Windows RT 8.1, Windows 8.1, and Windows Server 2012 R2 Update April, 2014:
*Included in 2919355 is this statement:*
/Important /*/All future security and nonsecurity updates for Windows RT 8.1, Windows 8.1, and Windows Server 2012 R2 require this update to be installed. We recommend that you install this update on your Windows RT 8.1, Windows 8.1, or Windows Server 2012 R2-based computer in order to receive continued future updates.
We have until May 13th to get this 700Meg plus file installed on Windows 8.1 otherwise you will not be offered up security updates in May.
If you are running 8.0 you will still get security and non security updates installed.
If you have access to TechNet or MSDN download and test it now. Come next Tuesday it will be needed update to your 2012 R2 servers and 8.1 workstations.
Windows 8.1 Update: The IT Pro Perspective: