archon saidits been done to me many times, and every time i forgive, meh.
That is why it keeps being done to you. By letting them get away with it, you are teaching them both: a) betrayal is no big deal, and b) you don't deserve better.
Soulasphyxi, did s/he come clean or was it a shocking discovery? Or did it happen right in front of you? All factors to be considered.
Considering I don't believe in trust in any case, there's not much for me to lose, but betrayal gives you an insight into his/her character, which I take into account in deciding if I want to know him/her anymore. I believe "There's a situation for every person in the world wherein he or she will betray you".
I forgive for them, only if they apologize and would actually do it differently if given the chance. I forgive for me if they don't regret it but I don't want to bother with being angry anymore. Forgiveness isn't the same as going back to them though. Self-respect is more important than any friend to me.