We've been implementing a way for users to post via email. Now, if you subscribe to a forum or a post not only do you get emails with the post information but you can respond to the post simply by replying to the email.
One issue we're having is the timeliness of the post. The goal is for the post to be posted no more than 5 minutes after you send the email out. Right now it may be many hours later. We're tweaking this.