I don't have the scriptures here in front of me but I do remember something about an age of accountability. Basically, infants, small children, those who haven't yet reached an age/mental capacity to understand what's going on are held accountable.
Geoff: Basically, (mind you this is how it's been explained to me) He is a loving and forgiving God, however, if you choose not to believe after hearing the truth, you have rejected Him and kinda sealed your own fate. God analogy: The Gospel is a life preserver thrown out to a drowning world, you can choose to take hold of it and live or reject it and drown. There is no swimming for shore.
As far as for those who haven't had an opportunity to hear and are past the aforementioned age of accountability, its supposed to work that everyone will hear at least once. That's why if you hear and it resonates, its not wise to put it off, it could be the last chance you have. However I struggle with this one myself...Surely there's unknown tribes in Africa or the Amazon, etc that have never heard the Word and are dying on a regular basis. So, that's the best answer I can give you.