As quite a few responses have put forward, I agree that belief in Heaven/Hell comes down to perception.
Some people see this physical existance as a utopia, they are happy with how they are treated here, and how they can get away with (mis)treating others. They are happy because this is all they know, trapped in their short-lived illusion with seemingly satisfying occurances.
As mentioned by Satori and others, religion is a tool used to control the masses. Unfortunately there are some genuine people who are trapped by the pressure and rhetoric splurged out by the religions, who use half-truths to form complete lies. Our minds have been bombarded and closed off so much, that most of us only perceive this physical illusion, where we supposedly 'work' most of our lives to grow, only to have it mean nothing in the end.
Also, the extremes of Heaven/Hell help the churches to commit you either way, you fully believe in the church, or you don't...which leads to: you have access toheaven if you believe what we say, or you are condemned to Hell if you don't. They can't allow you to believe in them a bit, because then they don't have total control over you, you become an enemy to them, a free-willed thinker.
As Trapped and HoserHellspawn started to touch on, Existance is more than 3 scenarios, being Earth, Heaven and Hell. There are more dimensions in between, where Satan/Devil/Evil/whatever has control over those that aren't purely divine. Satan is not on the same level as 'God', he is of a lower level, and has cut off levels below him. Which leads to the idea that those he does control are in somewhat of a Hell.
Some ask why 'God', being omniscient and omnipotent, can't just click 'His' fingers and get rid of Evil. If 'He' were to do that, our comparatively feeble minds would not be able to comprehend it, and would be damaged incomprehensibly by the event.
As Satori points out, religions imposing fears on their followers has major influential power. To stop and think for a second, by definition, 'God' is a being greater than humans. For 'Him' to be so insecure, to 'Himself' be so fearful that 'He' has to rule by fear, is just a totally innane concept. Maybe it leads to a socially unusual concept, that the 'God' of the Bible may not be all powerful as described.....maybe it is that the 'God' of the Bible is actually an usuper in diguise....
To sum up this current discourse, I see that this world is degrading faster with each passing hour, while the illusion of progression clouds our minds. Even if there isn't a Biblical 'Hell', we are certainly, as a world community, making one for ourselves.
All the above, In My Humble Perspective.
E.S.
P.S.: My sincere thanks to SaintJoshy for starting this interesting thread.