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.

So, whether or not you actually want the keyboard and mouse improvements provided in for Windows 8.1 Update 1, you'll need to install the update no matter what.  If you don't, no further security updates (or non-security updates) will be made available to those computers not running it. This seems to suggest a very tiny patching window will be granted to organizations running Windows 8.1 because once Patch Tuesday hits for May 2014, non-updated computers will become non-compliant and non-supported – effectively putting Windows 8.1 (and Windows RT 8.1 and Windows Server 2012 R2) into the same unsupported state as Windows XP (which reaches end of life on April 8, 2014).

Windows 8 (without the 8.1 update) is fine and will continue to receive updates until January 12, 2016.

Also, don't forget that Windows Server 2012 R2 will also receive a similar update and without it, Windows Server will also stop receiving updates starting during May 2014's Patch Tuesday.