That is something that have happend to me alot, and thats why I always copy all the written text in the box before I hit submit.I also support this idea...especially when I'm typing a long post and click "Post Reply" only to find that I lost everything because of a connection error or some sort.
This is a great idea but it would probably be more efficient just having the localstorage feature for automatic draft saving in case of an error and then maybe a manual save-draft option that utilizes the server for your idea....You might be able to use the browser's localStorage for this, but it would be much more convenient if I could save a draft in one browser and open my account in another and have my draft available. I'd rather have a separate "draft" table where the draft was stored server-side. Drafts would save the message, forum you were writing it in, the post you were replying to, etc. You could view/delete/pubish your drafts from a page in Your Account. Have an admin-set max number of drafts that each person can have (default to about 10). If you have a new thread draft, it will show up in another color at the top of the thread list in the forum you were writing it in so you can click on it, edit it, submit it. If you have a draft for a reply in a thread, an extra notice is added to the forum view, indicating you have a draft in the thread. It would also show up in another color at the end of the thread. (Only one draft per thread) Of course all drafts would also be available from the Your Account page.