The last time I played a good iPhone app I lost a couple fingers. There are so many good zombie games out there – Alive 4-ever, Zombieville USA, Attack from the Dead, Zombie Attack!, Zombie Invasion, Zombie Nombie, SurrounDead, etc.
Personally, I’d go with Zombieville USA, but PocketGamer.co.uk can give you a rundown of all the best ones.