Hi & thanks for pointing in the right direction, however, the solution above messed up the number of posts being displayed in my single post view. I resorted to the function used in the post-count plugin and added a category 'filter':
<?php
function mdv_post_count() {
global $wpdb;
echo $wpdb->get_var("SELECT COUNT(*) FROM $wpdb->posts WHERE post_status = 'publish' AND post_category = 'your-category-name' AND post_date_gmt < '" . gmdate("Y-m-d H:i:s",time()) . "'");
}
?>
Call the result in your template using <?php mdv_post_count(); ?>
(I didn't use the plugin but pasted the function into a functions.php file in my theme folder. You could duplicate and rename the function for different categories)
Anyway, figured this might be helpful to someone. For a bit more completeness there's also the msg-count plugin which you could modify the same way.