http://dayzmod.com/index.php
yes, it's a mod (based on ArmA2 Combined Operations), yes, it's still in the alpha stage, but already available to public. it's a bit rough on the edges, but it has great potential
I'm not going to try and convince you to buy ArmA2 just to play this mod. but trust me; when you'll launch it for the first time, you'll shit bricks