It doesn't work because in the default theme the line for display options in contact list select is like this :
<option value="{$contact.id_contact|intval}" {if isset($smarty.post.id_contact) && $smarty.post.id_contact == $contact.id_contact}selected="selected"{/if}>
If you replace the $smarty.post by $smarty.get, I think it can works with something like this :
<option value="{$contact.id_contact|intval}" {if isset($smarty.get.id_contact) && $smarty.get.id_contact == $contact.id_contact}selected="selected"{/if}>
Or if you want you can replace by $smarty.request so it can works with POST and GET methods.