Zend_Feed_Reader_Collection_Category::getValues() currently returns an array of 'label' values. This makes sense as the label element (according to the Atom Syndication Format specification) is supposed to be human-readable. However, the label attribute is optional (again, according to the Atom Syndication Format specification) and some feeds instead use the required 'term' attribute. Consider adding the 'term' attribute to the returned array instead of the 'label' attribute if and when the 'label' attribute is an empty string. For reference:

http://atompub.org/rfc4287.html#element.category

As a side note, this new category functionality is working very well!