Is Heaven Eternal?

A couple months ago, I was asked to give some quick pointers on responding to a person's issues with the existence of eternal punishment in hell. The answers I provided scratch the surface, but seem good enough to post here because they might get someone thinking about the denial of eternal punishment from both a Scriptural position (for the Christian) and a philosophical position (for the non-Christian). The person who was challenging is not a Christian but is willing to accept that Scripture is the source for orthodox Christian belief. The interesting implication that I point out is what inspired the odd title of this post.

Here is the email that I sent (I've edited it a bit for clarification):

Responsibility to Know and Act

Can God hold us accountable for what we are supposed to know is true?

I have two thoughts on this subject:

1. Analogy from speed limits. You have a duty to know where the speed limit changes, especially if you live in the general area. If a speed limit sign is temporarily obscured from view, you are expected to have seen it in the past, or know by other means what the speed limit is (such as from friends familiar with the area or context [neighborhood, school zone, highway, gravel road, etc...], even if the context may include multiple limits, acting upon the lower possibility will eliminate the possibility of being pulled over, while acting on a higher one puts you at risk of being pulled over). You are expected to use these other means until you know for sure.. If you are caught above the speed limit (regardless of reason), you are guilty of acting against the law and should be punished according to the law. If the cop wishes to show mercy, he may by issuing a warning. If the judge wishes to show mercy, he may either cancel the ticket or reduce the fine. Either way, it still stands that you broke the law; however, it is within the power of those who enforce the law to show mercy.