With Day-Z out there a developer has just announced their own open world zombie survival game.
IGN article (http://uk.ign.com/articles/2012/07/19/the-war-z-a-new-zombie-survival-experience)
Nice that they were going for the unlock areas approach but decided to choose open world with additional maps to be added for free later.
If this has truly been in development since last year, I think it's more in response to Dead Island and its follow on title.
Still Q3 2012 isn't too long a wait to find out.