Microsoft has now gone ahead and released the KB Article for Windows 8.1 Update 1 ahead of the April 8, 2014 public availability.
You can read the full KB Article (2919355) here: Windows RT 8.1, Windows 8.1, and Windows Server 2012 R2 Update April, 2014
However, there's one very important piece of content in the KB Article that really needs its own special highlight. In the Introduction section it reads:
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.