I can successfully display the comments form without an iframe by substituting line 253 in jetpack\modules\comments\comments.php (public function comment_form_after):
<iframe src="<?php echo esc_url( $url ); ?>" allowtransparency="<?php echo $transparent; ?>" style="width:100%; height: <?php echo $height; ?>px;border:0px;" frameBorder="0" scrolling="no" name="jetpack_remote_comment" id="jetpack_remote_comment"></iframe>
echo wp_remote_retrieve_body( wp_remote_get($url) );
It works too, I can submit comments just fine, though I do notice that the resulting form action is pointing at my local wordpress comment script (http://mywebsite.com/wordpress/wp-comments-post.php) rather than at http://jetpack.wordpress.com/jetpack-comment/ . Also it's loading the whole page with meta tags and all (head and body tags are stripped out, maybe by Chrome? or by wp_remote_retrieve_body?). In order for it to become a clean solution (and a step towards allowing posting comments via ajax), perhaps http://jetpack.wordpress.com/jetpack-comment/ would have to generate an adequate response for such an external request, without all the head links and scripts and meta tags...