Kaodi, I have two problems with allowing magic. The first is that it takes away from the "Average Joe"-ness that the PC's would have. If you can conjure fire out of mid-air or knit a broken bone by touch, you simply aren't average. Part of what I liked about this idea is giving them a feeling of vulnerability.
The second thing is that magic is a powerful and easily renewable resource. A good part of the terror in a survival/horror themed movie or game is centered around the lack of available food, weapons or other necessary resources. Imagine
Dawn of the Dead if everyone morning there was a beam of light in the mall and pearly white shotgun shells descended from heaven. It's be a boring movie if they had a divine source of aid that couldn't be permanently emptied. I could see the PC's going out, having an encounter, saying "Whoops, the Adepts out of spells" and retreating to a hidey-hole to rest. Now, you could stop them from resting somehow so that they didn't regain those spells, but in doing that the end result would be the same as if you had disallowed magic to begin with.
I can understand why people would be a little leery about that. Magic is a big part of the average D&D game but I could see it putting a crimp in the mood of the campaign.
Satori, I like that but what I might think of doing is adding them in as a leaner, hungrier zombie. Maybe the characters see a huge horde in the distance and figure it's not a big deal because they can easily outrun the shamblin corpses. All of a sudden a number of zombies jerk as if being pushed out of the way by something else, something quickly shoving and tearing its way through the crowd.
Freebfrost, I'd love to play that but I only have a PS2. D'oh!
Maybe I can read up on it and mine it for ideas.