In case you haven't noticed, Duesentrieb's category tree extension is live on all Wikimedia wikis. The subcategory list on category pages is displayed as a tree by default. You can click "show as list" to show it in the old display format. This additionally sets a cookie so that further requests will show a list by default (except pages cached as a tree on the client).
The suggestion has been made to add a user preference in addition to the cookie mechanism. I think we'll probably wait and see what the demand is for that.
-- Tim Starling