Somebody called Rob Bell (who I know nothing about although everyone else seems to) has started a discussion on the existence (or non-existence) of hell. It's gone viral all over the Christian blogosphere in the last 24 hours. I would recommend that you check out Sam's post at ELIZAPHANIAN if you want details.
This is my two-penneth:
When we talk about heaven and hell we tend to ignore the Incarnation. We are part of the body of Christ. On the last day we will be judging ourselves. This means that we will be accountable to those whom we have sinned against.
I have always felt extremely uncomfortable on those occasions when I have considered the reality of hell. I feel like a murderer designing my own prison. My conclusion is that it is only those for whom we have made life hell who have the right to deny the existence of future hells.