I think this is a great idea. I would have many uses for scheduled posts. The way IPB does it would be fine (at the time a thread is created), but I would also appreciate the ability to add posts to existing threads according to a schedule in the future. With this feature, admins could create a bunch of new threads they know will be of interest to readers in advance when they have more spare time. It would also allow admins to "bump" various threads with items of interest on a regular schedule to keep them alive.