I thought about running such a campaign once, but gave up without announcing it to the players.
The issue came down to "fightin' the zombies". In most zombie movies, the heroes use guns, but sometimes use melee weapons. Movies aren't a good example though, since the characters in the movie aren't under their own control.
Of zombie games, I'm most familiar with Resident Evil. In that game, if you actually get to use melee attacks, they're wimpy. You can't just take on large numbers of zombies as you'll run out of ammo. (In RE4, you can't buy ammo at the shop. Grrr...)
A friend of mine told me about another zombie game where you don't get to use guns. Instead, you have to get close to the opposition or run.
In D20 Modern, you'll likely have a mixed party. Pretty anyone can use guns, and some can use them well, while some will have Melee Smash, Archaic Weapons Proficiency, and high Strength, using weapons that dish out as much damage as guns (or use Combat Martial Arts, a weapon that literally cannot be taken away). Running out of bullets won't scare these PCs all that much. Dealing with the resources (bullets, maybe grenades) isn't in my personal skill set, especially if it only affects some of the PCs.