DayZ just keeps growing. As I'm writing this, the official website shows that 948,610 unique players have found their way to the zombie mod, of which 188,814 in the last 24 hours. Earlier this month the number of unique players was 'only' at 500,000.
The recent Steam Sale, as well as DayZ youtube videos ensuring hilarity and exciting times could be responsible for this notable increase.
You will need ARMA II and the expansion Operation Arrowhead to play this. Both are available in ARMA II Combined Operations, which got a great discount during the Steam Summer Sale. Exactly what those who still doubted needed to start playing this great game.
The makers of DayZ are working on a standalone version, meaning ARMA II won't be needed anymore. Seeing as the mod is still in beta now, a lot more content and tidying up will be needed; yet they are optimistic it will be released before Christmas, following the Minecraft model of a low price point.