Interesting you point this out. This has been on our road map for quite a while. The only caveat is in the implantation we considered the author would only be allowed to delete the post if there hasn't been any responses to it. In any case, I will have the team revisit this feature.
I think we are on the same page. In your example, you as the respondent would have the ability to cancel/delete your post as long as no one has respondent to your response. If we allowed users to delete any post that had a response then all posts that occurred after your post would also be removed from the site. Does this make sense?
I think there are two "workable" scenarios here :) If you are the OP you can only remove your post if no one has replied. If you are replying to a post that you did not originate, and you throw up some crazy answer (like I'm want to do ;-) then you can pull the post as long as it hasn't been rated (good or bad!)
I know that I've prematurely posted questions out here, and then later fixed the problem. I hate to waste people's time, so I just edit and then type "deleted" in the body of the message. I completely agree that was should have some way of canceling messages that we've posted. =)