It was available via Steam Early Access on September 20, but will have a full release on Tuesday, November 5.
http://undeadlabs.com/2013/10/news/steam-full-release-tuesday-november-5/
Undead Labs said:Thats right, were heading into the homestretch. Of course, well continue to make improvements, but were just about ready to call State of Decay launched on Steam. The official opening day will be Tuesday, November 5.
Were going to do some serious celebrating on Tuesday. Prizes and contests and more, oh my!
In the meantime, if you are one of our adored Early Access players, there is a request list thread in both the official forum and the Steam forum. We sure appreciate the help!
http://undeadlabs.com/2013/10/news/steam-full-release-tuesday-november-5/