The term_group field is notoriously under documented (wasn't even in the first taxonony proposal http://www.nabble.com/Taxonomy-Schema-Proposal-td10011364.html )
I could think of many uses for this field, including a plugin I'm currently coding, which need to distinguish between two 'types' of categories ( and where a 'categories custom field plugin' would be a little heavy )
But there is no way to query categories on this field !
I search wp-includes/taxonomy.php for a way to add a where clause on that, but the only mention of term_group is to order the query by term_group
Any thoughts on how term_group could be used ?