I cant see the {cat_id} placeholder in any of the layouts. Is this a mistake or or done on purpose? I consider that a bug, hence I put it in this category. That {cat_id} could be used as a css id which would allow for category specific styling which is otherwise not achieveable.
Hm... will check this.
It seems that the placeholder is not listed in the layout description. But you can use him in some layouts with {cat_id}.