My basic thoughts on the subject are that hoaxes like this mean that if something really crazy and nerdly did happen, Zombies, Aliens, Demons, invasions of mole people from the center of the earth, etc... nobody would take them seriously due to all of the people making hoaxs like these.
As someone who has worked security before, I will say that the biggest problem in an emergency is not the danger itself, but stupid people who refuse to act or react unless they actually see a danger themselves. This being anything from a fire, to a chemical leak, to anything else. People are more worried about being embarassed, taken advantage of, or exploited due to hoaxes than the potential dangers involved.
Case in point, you have a fire back of the house in a service/mini-kitchen. While the fire response goes out, security heads out to discretely remove people from nearby gaming areas BEFORE the fire could potentially spread and do some damage. The response from your average person is "we don't want to do anything unless we see it" and it's especially a problem when you consider that information control is important, you don't want people to even know what the emergency is for a number of reasons, including potential overreaction (the flip side of the coin), or even law suits (claiming smoke inhalation even if the fire never got that close to you), and similar things. I've in the past thought we should have been issued truncheons and the authority to beat people unconscious and have them carried out on stretchers if they didn't want to evacuate since it's less potentially dangerous than say being burned alive, or killed by some idiot who mixed Ammonia and Bleach or whatever, and the more time you spend argueing with people the less effectively your going to clear an area... and of course you don't want word of mouth to have people on the other side of a building leaving or overreacting (potentially) when it's a localized problem, and you just want people out of a specific area.
The point of that aside, the bottom line is that we've already trained people to not take alerts seriously, and not to respect the authorities who should know better. Hackers and such do not help the situation. Ideally when you hear an emergency alert, or the guy in the blazer with the security tag says "do this" people should do it immediatly, which is the point of why you train in drills starting with school. Of course arguably those same drills combined with the hackers and such have contributed to people actually becoming stupider and less willing to act. When things don't work there are of course criticisms like "durrr, why didn't you clear people ahead of the fire before it spread" or whatever.
It sounds twisted, but I think you'd actually clear people faster with tasers, truncheons, and stretchers, especially when other people saw the process and learned to take it more seriously (I'll leave it to your imagination how literal I'm being here... ) :)
At any rate, my basic attitude is that if someone DID make an alert like this, people would ignore it, and that in of itself would contribute to something like a Zombie outbreak. Likewise if aliens like the ones from the horrible movie "Skyline" appeared and the authorities announced "aliens are attacking, they have a visually based mind control device, do not look outside!" 90% of the people would immediatly look outside. :)
Of course at the same time, while I'm likely to evacuate if asked by the authorities professionally, having been on the flip side of that coin, if someone did annouce something really incredible I'd admittedly look for confirmation by multiple sources. For example if on TV I got an emergecny broadcast about zombies, I wouldn't start gearing up for the apocolypse until I saw multiple sources, including actual news anchors at least relaying the information (preferably on more than one network).
As far as what I'd do in the case of a zombie apocolypse? Rather than talking about my "mad skills" and how genere savvy and able to survive I am, I'll just say I'm a horribly, increasingly, out of shape nerd today, I've been destroyed by brain damage, live on social security after what amounts to forced retirement, and am at the mercy of the modern medical establishment for medication and such. I'm bitter and generally tend to hate everyone and everything for a reason (grr!) and am honest enough to admit not only that, but that in the case of some awesome nerd-fiction come to life I'd hardly be the guy to thrive in that enviroment. I'd probably do okay until my meds ran out, then well... it would be less okay.