Okay, I'm a bit confused by the normal forced movement comment.
In our current 4e campaign, we've been playing it that any forced movement (push, slide) over any kind of ledge or cliff that involves a drop, gives the target a save. If they fail the save, they go over. If they make the save, they stop on the last square before the drop, and are prone. This makes sense, because as they were going over the ledge, they grab on, hold, and pull themselves back up onto the last square and they are prone. You couldn't pull yourself up over a ledge, and straight into a standing position in one move.
Is this not right? Thanks.