The public demo will be coming soon, one of the testers leaked the code on 4chan so the devs released it early. I just played it and besides some minor graphical glitches it looks to be an awesome game in the making. Oh and it's hard, played twice now and both times I died before I even left the front yard.
http://projectzomboid.com/blog/index.php/2011/05/alpha-tech-demo-released/
http://projectzomboid.com/blog/index.php/2011/05/alpha-tech-demo-released/