I'm kind of surprised I didn't post this already (maybe I did and I just can't find it) – but on another blog, I have drop down comments set so it only does this for the first 'x' number of posts with comments. (Otherwise download times can get a little bogged down if there's a lot of posts with comments on the page…) To do that I have this at the top of my page:
<?php
$showcommentmax = 3;
$currentshowcomment = 0;
?>
Then I have this code around the drop down comments call: