Hiya.
We actually "just" (three sessions now?) started a zombie-apocolypse game. We looked at Rotworld, AFMBE, and one other I forget. After giving the rules a read-through, I decided I didn't like any one of them. Aspects of each I did like...but none "fit" with what my group was looking for. In the end, I decided to use some of the ideas from all of them, but use the Top Secret S/I rules.
The TS rules have hit locations, so that worked out to what we wanted (head-shots to take out zombies, and locations to blow off zombie arms, legs, etc. and still have them crawling after you!

). I used a "biological reason" for zombification (not cosmic radiation, magic, prophetic, etc.; my version has it starting after a virus that wiped out the dinosaurs finally made its way through the ecosystem after all the polar ice cap melting and such).
I had to come up with some "zombie mob" rules, which are actually quite slick, if I do say so myself. Zombies are fought 1:1 up to about 15. Once you hit 16+ zombies, you have a "hoard". A hoard has a set average head HP (typically 5), move, damage, etc. During the initiative roll, any player who's initiative die (it's a d10, btw) comes up the *same* as the zombie hoards, indicates a "zombie surge"; the zombies all get to move twice their normal move that round, and I roll 1d10 and add the number of the initiative; these are extra zombies that show up "out of nowhere" ...you know, when you have just heard a noise in the back staff room of the supermarket you are ransacking...and a zombie walks out, as well as a dozen more from the deli, produce section, etc...you fight back out towards the front doors and *blammo* another 8 zombies are milling through the parking lot towards you. Where did they come from? You walked through the parking lot and nobody was there? Nobody in cars... nobody on the streets...wtf?!? Well, in a zombie flick, this kind of thing happens all the time. A zombie hoard is like that.

(for an example, go watch just about any episode of The Walking Dead...they're all standing on a highway full of cars wondering how to get through...minutes go by...then a zombie comes out from around the RV....Aaaaaa!!!!...a shot rings out, camera pans back and...there are zombies shuffling their way through traffic....how did they *not* see this?....I'll tell you...*zombie hoard rules* baby!

). The other nice thing is that it encourages "small groups" to try and sneak into town and whatnot...less people, less d10's for initiative, less chance to match the zombie hoard initiative.
Anyway...the PC's have managed to secure some transportation (a ford Focus and an old Lincoln Town Car) from two farms about 30km north of Prince George, BC, Canada. I think they are planning to head north...less people = less zombies...or so they think! Other people had that idea too...*thousands* of them, and they flew in before all hell broke loose. Should be interesting when they get to Fort Nelson and find an extra few hundred "people" there (Ft.Nelson has a population of about 4,000 normally...).
^_^
Paul L. Ming