<blockquote> I decided on the application-determined one-notification
approach, because that's basically how the old bboard
worked (if you had thread notifications, you wouldn't
also get the forum notification, though I suspect you
might actually still get the batch notification if you
had that ...).
</blockquote>
Actually, I could be totally wrong, but as I recall, with the old bboard, you could decide whether or not to receive notifications for a given thread, and you actually could receive double notifications if you were subscribed to both a thread and a forum. I seem to remember this happening to me.
In the case of forums, it might make sense to provide a checkbox (or link) on a thread page which is unchecked by default when the user is subscribed to the parent forum, and checked when the user is not subscribed. The principle can be generalized from there, but the basic idea is to provide an API so that it's easy for an application to do this sort of thing. The application may have to do a bit more work, but atleast everything remains flexible.
Hope that made some sense...
