At this points $current_user is not created yet. at least that, what it looks like. you can get user_login directly from wp_login action like this:
function access_log_insert_record($login)
{
global $wpdb;
$table = $wpdb->prefix."access_log";
$wpdb->query("INSERT INTO $table (access_user, access_date_time) VALUES($login, NOW()) ");
}
add_action('wp_login', 'access_log_insert_record');
At least, thats how i got user thats loggin in from wp_login hook.
