Hello, Thematic masters!
Hope this is not a false alarm or a PEBKAC. :-D
I have comments disabled via Dashboard, Settings, Discussion, but enabled for a particular post (via the Discussion metabox of that particular post). Thematic says comments are closed for that post. You can see and test here:
(This is a test site that runs latest trunk of both WP and Thematic — just did an svn up.)