get_post_format_image_src()

/* In functions.php */
function my_theme_get_post_format_image_src($post_id){
    $format_meta = get_post_format_meta($post_id);
    $match = array();
    preg_match('/<img.*?src="([^"]+)"/s', $format_meta['image'], $match);
    return $match[1];
}

/* In the template file */
$image_src = my_theme_get_post_format_image_src($post->ID);