you're over complicating things, just do this:
Code:
<?php if(get_post_meta($post->ID, 'author-comment', true)): ?>
<div style="somestyle"><?php echo get_post_meta($post->ID, 'author-comment', true); ?></div>
<?php endif; ?>
then just use a custom field named author-comment (or whatever) with the text you want and apply the style as you want.
Alternatively, if your theme doesn't use excerpts, then you can simply place the_excerpt() there, so you'll only need to add an excerpt to display that text, and if you don't input anything, then nothing will be displayed.