here's the changelog from Rocket:
DayZ Update 1.5.7
UPDATE : 14 MAY 2012
* dayz_code 1.5.7
* dayz 1.2.4
* dayz_sfx 1.1
* ITS MY BIRTHDAY. I AM 31.
* [NEW] Zombie Script converted to Finite State Machine for performance improvement
* [NEW] Zombie Numbers drastically increased
* [NEW] Amazing Draft Zombie sounds provided by Michael Manning (http://www.manningaudio.com
* [NEW] Terrifying Zombie face provided by GenesisCrys (http://www.lostpoly.com
* [NEW] Panicing sound effect
* [FIXED] Death when entering crashed heli's
* [FIXED] Hardly any zombies spawning (Lol... uh... like there is lots now)
P.S. Re: Zombie Numbers.... hahah... you're all #%&*$#. deeply #%&*$#. Good luck.