According to developer Undead Labs on the official blog, "Greetings, PC players! Our Early Access version of State of Decay hits Steam on Friday, September 20th."
The price is twenty bucks, same as the Xbox 360 version. Those willing to throw in with us at Undead Labs and your fellow early adopters will automatically get the final version, completely free, with no need to restart, when we officially launch later this year."
As it is with all Steam Early Access, the game is in beta form, and as such, there's no keyboard/mouse support - only gamepad. The final version will support it, but for now, an Xbox 360 gamepad or compatible third-party equivalent will do.
"This process is not for everyone. Casual players, the easily frustrated, and the short of time should NOT get the Early Access version," warns the developer, "Hey, I am all three of those things, y’all. Those are not criticisms. It’s just that we want everyone to have the experience that best suits their needs. You all deserve the best, so if you’re not into giving feedback, just wait a few months and play the final version after we make it great. And it will be great, with the help of the rest of you — the hardcore early adopters that made State of Decay into something huge."
In the meantime, a sandbox mode is in the works for the XBLA version, which is sure to make its way to the Steam edition too.