Tell them! If you tell them, and you also tell them what you want out of the game, then they can work with you. If they're just going to do their own thing instead of working with the GM, well, why are you running for them to begin with? Say you want a grim survival horror game, and if they don't, they can just say as much.
I dislike horror in all forms; written, filmed, and role-played. If you told me in advance that you wanted to play a horror campaign, I would politely step aside and allow the rest the group to play.
If you told me it was going to be a cyberpunk epic, and it turns out you lied to me, I would probably be less polite. As in, feeding your module and sourcebooks into a shredder. Hopefully paying several hundred dollars to replace your materials will teach you that we are players, not servants.
I'll tell you this right now, me and anybody else I know wouldn't be the ones paying for any property you destroyed.
There may be assault charges involved over a few hundred dollars worth of property though.
But that's just me. Maybe where you come from people roll over and take it from petty little b#%!%es who flip the table and set it on fire if everything doesn't go their way.
I dunno. How well do you know your players?
Personally, I'd be kind of excited to have a little mystery injected into my game. I'd be fine walking into this kind of scenario with minimal info. :)
If you think everyone would be fine with a survival/horror themed campaign, and aren't afraid of playing characters that might not be completely optimized for the sitch, I'd say cry havoc and let slip the zombie apocalypse without warning!
If, on the other hand, you have someone who you know thinks the zombie thing is so over, or have players who really prefer their characters optimized for a campaign, you might want to discuss things a bit more beforehand.
This is always a touchy subject.
I personally hate zombie apocalypse stories. If I knew it was that kind of game, I'd bow our. But if it was just known to be gritty low level - I might buy in. Then when the reveal happened my general reaction would be "This isn't the campaign I signed up for" and bow out, but feel a little annoyed.
The surprise can work, but I am generally opposed to any kind of bait and switch kind of set up.
But in these circumstance knowing your players is the most important thing.
Do not tell them its gonna be a ZA. why? Because the players will create the best builds that they can to survive it even if the setting is a place that has very little to no undead in the area, each one will build a charecter that can do something to undead and that pretty ruins the whole reason for ZA theme.
BUT let them no its gonna be a low magic, low supplys, more focus on survivability and let them create their charecters. Once they have created their charecters, makes scenarios that spotlights each charecter. A sword and board fighter? Create a scenario where hes protecting helpless people against hordes of zombies. The dreaded smooth talker focus charecter? Create acenarios where places are inaccessable because people are fearful of the creatures and outsiders and losing supplys to other people and have that person be the one who got them into a safe area whereas without him, they woulda been stuck in what seems like total wipe of party situation.
Play to their strengths and weaknesses, make them think, make them react, make them desperate, but dont be heavyhanded in the hopelessness in that its gonna be VERY VERY difficult for them to survive.
Again dont tell them but play to the strengths weaknesses of their charecters.
The fighter who can mow down the zombies geta barred and frustrated that the fear of the people he wont be able to reason with most of them.
The talker who seems outta place in combat becomes the one who was the only peraon who talked that peraon into their house, or found a shortcut, etc etc.
the playera dont have to no exactly what its about as long u the dm give their charecters moments to shine and be important. Make it to were the fighter sees the smooth talker as someone they need and the smoothtalker see the fighter as someone they need.
If u tell them, chances are they will metagame and it will take away from the impact of alot of scenarios. But just my 2cp. Man would love to be part of that group lol