For those with XBOX 360's there's a demo available to download from XBL Marketplace. Played it last night and from what little there was in the demo, it's fucking great.
I was in a shopping centre with about 500 zombies, all lumbering around and I could interact with lots of things in the environment, such as breaking shop windows and taking items from the shelves. Changed my shoes to a pair of trainers in a sports store, but I didn't seem to run any faster.
Some zombies (such as security guards) carry weapons, so when you kill them you can use the weapon yourself. You can even pick up body parts and either use them as weapons or throw them as a distraction to the undead. If the final version has all this, plus a tactical/story driven purpose, it'll be something else.
EDIT:
http://sharpfish.realityfakers.com/ review of the demo.