Dynamically Determining Category or Creating Dynamic Category lists

I’d like to create dynamic list of categories & their children using WP 2.0. e.g. initially the sidebar shows

* Fruits
* Vegetables
* Minerals

If you click Fruits, you get the Fruit category archive page and the side bar now looks like this:

* Fruits
– Citrus
– Stone fruits
– Berries
* Vegetables
* Minerals

BUT I (A) can’t seem to find the way to determine what the current category ID is. And (B) wp_list_cats doesn’t seem to want to list just the top-level categories plus the sub-categories for the specified parent.

I would prefer to not hack the core WP code…