I tend to agree with most of what's been posted so far.
I think forgiveness depends on what exactly you have to forgive. if you know somebody didn't do something on purpose, then it comes a lot easier, well, that's a given.
However, if it was deliberate, then I would have a hard time forgiving anyone, depending on the situation. Kit was right in so far that your relationship could never be the same, you would always be on guard against any further breach of trust. Talking from experience, I may have forgiven certain people, but I have never trusted them again the way I would like to.
It's only ever happened once, a long time ago I might add, but if a partner cheats on you, then I think it's a lot harder to forgive what I consider to be the ultimate betrayal. And no, some 7 years later I still haven't done it, we talk etc, but in my heart I know I just can't forgive him for doing what he did.