Correct me if I am wrong.
but cant you have it email you when there is a reply.
i realiz the differenc but it can accomplish the same thing.
P.S: Just make the thread a browser favorite
I believe the email reply is only for threads you post in, and he is looking for a way to find older topics to use a reference, not really to see if there have been any new posts. As for using browser bookmarks, having it be a feature of the site means that it doesn't matter what computer you use to read the site, your bookmarks would be there.
I think it would be possible, just store a list of topic ids for each user. Maybe even more interesting would be to store a list of user ids with each topic... that would allow for a way to browse "popular" past topics. The time consuming part in all of it would be writing the interface