The green bubble (status is enabled) next to the category items in the category overview suggests that you can disabled/enable the category.
Is that possible? And if so, how?
I don't think you can disable categories with the current version of Craft. But you're right, it currently looks like that categories are prepared to get disabled.
Categories now have their own edit pages in Craft 2.3, where you can set the status. And you can even set it on a batch from the category index page!