I am sure many of you have already found this bug and have squashed it somehow, but I had not done that. So if you are still one of those people using WordPress 1.2 Mingus and run intro trouble mass deleting comments, this is the quick solution.
Open up your edit-comments.php file which should be in wp-admin in your favorite text editor.
Find this line:
else $mode = $_GET['mode'];
And add this just below that line:
$delete_comments = $_POST['delete_comments'];
Now delete comments en mass with glee!
Sorry for the late response Narada.