While your changing how replies work, I have a small feature suggestion.
If a post being replied to is changed while someone is writing a reply, they should be notified. Possibly just match the changed date or an md5 checksum on the post when posting. If it doesn't match, just show the reply screen again, with the updated post and a nice big warning box mentioning the fact that the original post has been changed.
Even better would be to have a colored diff showing the changes, but that might be overkill.
This could of course be polled via ajax, to update the original post inline without having to wait for the submit.
