On another site forum I frequent, contributors are able to edit posts until such time as someone else responds. I think that's fair enough - if you could edit them after a reply, the person replying could end up looking rather silly, taking you to task for something that then disappears from your post.
The tricky thing about that is that the AB forum is in a "flat" style (replies are just added to a thread in chronological order) rather than a "hierarchical" style where each reply is attached as a follow-up to a particular post wtihin that thread, so you might be prevented from editing your post by someone replying to someone else's!
I still agree with smudge that an edit facility would be worthwhile though. Several times I've typed out something in response to a post that has annoyed me, only to come to my senses at the last moment, realise that posting would only make things worse, and hit the back button instead of submit. It would be nice to have the opportunity to withdraw a comment before response if I did post it in a fit of pique!