Subcategories of subcategories are not allowed it seems (shame, but there you go)
However while looking for something else I stumbled across a function isGrandparent() in discourse/app/assets/javascripts/discourse/models/category.js.es6 on line 68
It seems to be intended to return true if one or more subcategories have subcategories of their own.
Does this imply that nested subcategories are being worked on and will be here soon - or have I misunderstood its purpose?