That's what concerns me. There's probably enough nuts out there that might try to get there hands on a nuke or a bioweapon or something just to make sure doomsday happens when it's supposed to.
The so-called http://en.wikipedia.org/wiki/Reflexivity_%28social_theory%29 issue is a very interesting and complicated topic in psychology and sociology. There's also http://en.wikipedia.org/wiki/Self-fulfilling_prophecy on Wikipedia specifically about the "self-fulfilling prophecies" aspect of reflexivity. Also related is the relatively well-known http://en.wikipedia.org/wiki/Thomas_theorem.
If doom-sayers manage to strongly convince a significant enough number of people that the world is going to end, say, in ten years, then even though the world may not actually come to an end at that time, the panic, mass hysteria and general chaos caused by those who think it will could have such a significant effect on humanity and human society that it could be, from the humanity's perspective, almost as if it did come to an end.