Thesis 1.7 in /lib/classes/comments.php line 123:

class thesis_comment extends Walker {
	var $tree_type = 'comment';
	var $db_fields = array('parent' => 'comment_parent', 'id' => 'comment_ID');

The thesis_comment class extends upon WordPress code and provides additional functionality. It is not just calling WordPress functions. That in my opinion makes it derivative work, not simply dependent work, and therefore violates the GPL.

Update: Drew Blas has posted a far more in-depth analysis of GPL’d code in Thesis.