I didn't test too much if this is true, but I have a general concern editing postings:
Couldn't you disable editing postings that are 5 posts older than the last posting of the current topic?
You could easily offend anybody or have a conversation through ages and you could edit your first posting and everybody joining the conversation now would be screwed. And the worst part, those people lie to themselves AND to others.
I mean, you could change the point of a whole conversation from sillyness to a completely wrong picture.