I run a survival horror game with....
Zombies. That's right, flesh eating, wandering around and groaning, PC eating, head-shot only to kill Zombies.
I've already seen the "yawn" posts, and I'll tell you the truth, I haven't had that problem in the month I've been running this game.
While it isn't Fantasy d20, it's still a lot of fun.
The group has gone from a gaggle of unconnected people that happened to be in the same place to a tightly knit group determined to survive at all costs.
The players still get excited, the combat is still a blast (literally sometimes, if a wild burst sets off something combustible) and everyone has fun.
Here's the way I handle it. This is a Modern d20 game, heavy grit.
First of all, the players had NO idea what was coming down the pipe (The infamoust "TWO MINUTE SIGN UP WINDOW FOR AN ICQ GAME!" ploy netted me 9 players who knew little about each other) as far as the game, so nobody took feats, skills, etc geared toward post apocalypse survival.
Two: I used confusion. I won't allow a battlemap during modern games at IRL games (combat is confusing, nobody knows where everyone is, and many other factors), and love using vague confusion to hieghten the tension. On the IRC game we use PM's (I've fought TWO battles in PM only due to terrain/lightning, Only putting a few things in the main chat. Nobody has a clear view of what happened during those fights) extensively so that there is constant confusion.
Three: Ammunition, fuel, food. Going into a supermarket can be risky, as zombies congregate at places that were important fixtures in thier lives. Ammunition is scarce, but for the most part, it's living VS living battles that give ammo, but the sound of gunfire draw the zombies.
Four: The feeling of being outnumbered. With invasions, confusion and decimation of the civilian authority infrastructure, the total engagement of the military infrastructure, the is no law other than what the PC's can dish out themselves. The group contains 2 active duty military (One who was being transported to Fort Leavenworth to await execution for War Crimes, another whose helicopter went down) and a County Sheriff's Deputy (Who answered a "Shots Fired" call and ended up in a whirling nightmare of dead) who have difficulty with this part. For the War Criminal, this is a party, survival of the fittest. For the former NORAD Security, this is a nightmare where the world's potential is gone. For the Deputy, this is Hell on Earth. The party has NOONE to turn to, no place to run. All they can do is keep moving.
Five: Lethality. Yup. Pull no punches. There's no healing spells in Modern d20 Gritty, which means a bullet in the stomach is a bullet in the stomach for weeks, not a cure spell waiting to happen.
Six: Replacements. Usually, when people come into a game, or have to replace a character that got pulled down by zombies and messily devoured ala Day of the Dead, they get to roll up a PC of equal level. BUZZZ! No dice. First level. Of course, that's made it so others didn't want to join (For some reason I'm considered a Killer DM... <grin>)
Seven: Crippling injuries: We've got people who are badly burned scarred (FIRE BAD!), people missing ears, fingers, etc. ANY time a hit does max damage, something bad happens. We've got PC's whose ribs are taped up, and these guys are rarely, if EVER at max hitpoints.
Eight: Keep 'em moving. Don't let them hole up and heal up. Keep on em. One or two attack, a frenzied zombie bursting in the door and into the room. Fire, flood, natural disaster.
Nine: Don't be stingy with the gear, but don't be weak with the opposition either. How many Modern d20 groups have a Bradley "Linebacker" model AFV? We're talking 25mm Chaingun, M-240 machine guns, Ma-Duece HMG, Quad TOW-II launcher? Mine does. Know what it sounds like going down the road? "GAR-UMPH GAR-UMPH GAR-UMPH!" Yup, I've beaten the thing down. How? Rogue National Guard clashes against the PC's. So they have an Armored Vehicle, so what. The TC ducked back in, and the RPG hit the hatch and blew it clean off. Oops, can't seal the interior now.
Ten: Keep on em. Don't let em rest, don't let them regroup. Keep hammering at them. Old friends now zombies, family members scarfing down the remains of a kid, former team-mates shuffling along. Add in nightmare affects (Willpower Check: DC: XX) so they can't sleep. The moaning of the dead outside. A hit by a Rogue military unit.
Eleven: A goal, a place of hope. Always just down the road.
Twelve: If they do something stupid... Kill or maim them. Don't let them heal up too easily. I let the party rest for 1 week to heal up, on the third floor of a building after they destroyed the stairs, etc. Then, they had to try to get past the zombies, which could see them moving around... That was horrendous in ammuntion expendature.
Thirteen: Resources. If they can't carry it, it's gone. In a fantasy setting, that's spell components. If you don't ahve them listed, they are gone. That feat "Eschew Spell Components", forget it. It's gone. In my game, that's translated up to these guys policing up spent brass.
Fourteen: Keep shoving them. Have more zombies/ghouls/mutants/whatever on the way as they're searching the bodies. A "search" check takes awhile, and in the meantime, more reenforcements for the baddies are on the way.
Fifteen: Collapse of social order and services. No 9-1-1, no hospitals, no police, nobody waiting to rescue the characters. Bullet in the lungs? Too bad, better hope that doctor that joined the group is still alive. See that group of State Troopers? They are NOT here to help. In a fantasy setting, that means that the King's Men are now thier own...
Sixteen: Mysterious causes. What caused the disease. Nobody will EVER know. Flat out. Why did the Chinese invade? Who knows. Why did the Russian send paratroopers over the pole and into the US & Canada? Who knows. They're just here now. Who fired the nukes first, and why? What does it matter? By making some of the largest parts of the PC's lives affected by mysterious forces, it gives a sense of unease.
I could keep listing, but I figured I'd give someone else a chance to talk.
Hope some of this helps even slightly for your game.