sorting by custom field date

Hi all,

I want to show 3 posts that appear today or in the future.
My dates are stored as Y/m/d (so yyyy/mm/dd eventually).

I’m getting a rather random result right now, using:

$today = date('Y/m/d');
$args=array(
   'post_type' => 'Kalender',
   'meta_key' => 'datum',
   'meta_compare' => '>=',
   'meta_value' => $today,
   'orderby' => 'datum',
   'order' => 'desc',
   'nopaging' => true,
   'showposts' => 3
);
$wp_query = new WP_Query($args);

Any thoughts?