baffleinc

use a category foreach to get the category id, then add it into the query_posts() (add this just before the endwhile of the main posts:

<?php
	foreach((get_the_category()) as $category) {
		$category_id = $category->cat_ID . '';
		$category_name = $category->cat_name . '';
	}
?>

then in your sidebar:

<h2>Other posts under <?php echo $category_name; ?></h2>
<?php
if(have_posts()) :
query_posts('cat='.$category_id);
...

OR

<h2>Other posts under <?php echo $category_name; ?></h2>
<?php
if(have_posts()) :
query_posts('cateogry_name='.$category_name)
...

That should work fine. Just make sure the first segment of code is within the main Loop!