It doesn't work as seamlessly as other SMF forums, but the feature is there. It's the "Bookmark" option, a small icon at the top of each thread. Once you bookmark a thread, it is saved in your bookmarks list in your profile. Within that list, there is a link called "Manage Bookmarks" and you can then set up to be notified by e-mail of new replies on bookmarked threads.
Sounds like a lot of work, but once you have the e-mail notification set then it will be automatic whenever you bookmark a thread. Hope this helps!