[Plugin: Simple Fields] order post with custom field

hi,
your plugin is great!
i need to order post using the value in some custom field. is it possible or i could acces the value only in the loop?
this is the code i use:
$custom_field = …. ;

$args=array(
‘post_type’ => ‘my_post’,
‘caller_get_posts’=>1,
‘paged’=>$paged,
‘orderby’=>$custom_field,
‘order’ => ‘ASC’
);
query_posts($args);

thanks

http://wordpress.org/extend/plugins/simple-fields/