This was released as part of MDOP 2011, plus there is an RDS release ISO available.  The releases are available to MSDN Subscribers, and from the Global Volume Licensing site.  It formerly was also available via TechNet.  There are also upgrade only installers via the Microsoft Download site.

SP1 primarily was:

• A change to the Sequencer workflow.
• Support for read-only-cache client mode.
• Support for sequencing the .Net 4.0 Framework (not that we would recommend doing so!)
• Other items (see TechNet link below)

The Service pack identifies the following Hotfixes from the 4.6 RTM release as being included in the release.  Any subsequent hotfixes to the RTM are not included, but there probably is an SP1 hotfix that covers it available.

• KB 981787 (4.6 HF 1)
• KB 983173 (4.6 HF 2)
• KB 2307495 (4.6 HF 3)
• KB 2432513 (4.6 HF 4)

Some Additional Links:

• Download site upgrade exe: https://www.microsoft.com/en-us/download/details.aspx?id=14552 
• KB 2445990 Description of the release: https://support.microsoft.com/kb/2445990
• TechNet Information: https://technet.microsoft.com/en-us/library/gg507597.aspx
• TechNet Release Notes: https://technet.microsoft.com/en-us/library/gg507623.aspx
• TechNet release information: https://technet.microsoft.com/en-us/appvirtualization/bb508934.aspx