echo get_post_meta of all post in a category to fill up a select field

Thank you very much for your answers!

I finally found what I wanted here : http://sixrevisions.com/wordpress/custom-fields-search/
Here is the code :

<form name="search" action="" method="get">
<select name="City">
 <?php
  $metakey = 'city';
  $cities = $wpdb->get_col($wpdb->prepare("SELECT DISTINCT meta_value FROM $wpdb->postmeta WHERE meta_key = %s ORDER BY meta_value ASC", $metakey) );
  if ($cities) {
    foreach ($cities as $city) {
  echo "<option value=\"" . $city . "\">" . $city . "</option>";
    }
    }
 ?>
</select>
<input type="submit" value="search" />
</form>

This is a brilliant article and it helped me a lot! I hope it’ll help others too.