Windows 8.1 Update is now available via Windows Update, although it may take a while before it rolls out to your systems. See what's new and this blog post with more information. There is also a nice best of BUILD post you may find useful as well.
See KB 2919355
Note that this update is mandatory. All future servicing for Windows 8.1, Windows RT 8.1, and Windows Server 2012 R2 will be based on having this update installed. Windows 8.1 Update includes a roll-up of servicing changes to date. Windows 8.0 users also need to upgrade to Windows 8.1 Update to continue getting support as of January 16, 2016.
MSDN/TechNet Subscribers: This is available as both a standalone update and as a full Windows 8.1 ISO with the update included.
IT Pros: Here are some deployment details on the update, as well as some information on the Windows Server 2012 R2 Update.
Windows XP: In other news, as of today Windows XP is now end-of-life. See this blog post for my homage to this venerable OS...
WSUS Users: There are some issues with the Windows 8.1 Update deployment via WSUS. See this blog post and KB 2959977.
Windows 8.1 SDK: The existing Windows 8.1 SDK was refreshed for this update (April 2014). This refresh is also included in VS 2013 Update 3 or later. See MSDN for notes on the API changes.
Update: This blog post has some tips & tricks as well.