brickbrigade

OK, I figured out how to solve this a bit more elegantly. Add these codes to wp-content/themes/[theme-directory]/functions.php

add_filter('query_vars', 'metakey_queryvars' );
function metakey_queryvars( $qvars )
{
  $qvars[] = 'not_meta_key';
  return $qvars;
}

add_filter('posts_where', 'metakey_where' );
function metakey_where( $where )
{
    global $wp_query;
    global $wpdb;

    if( isset( $wp_query->query_vars['not_meta_key'] )) {
       $where .= $wpdb->prepare(" AND $wpdb->posts.ID NOT IN ( SELECT post_id FROM $wpdb->postmeta WHERE ($wpdb->postmeta.post_id = $wpdb->posts.ID) AND meta_key = %s) ", $wp_query->query_vars['not_meta_key']);
    }

    return $where;
}

What the above codes do is adding a custom parameter called ‘not_meta_key’. So now you can do this:

$myposts = get_posts('category=3&not_meta_key=feature-sidebar');