Memcached::OPT_SERVER_FAILURE_LIMIT - once a server has hit this limit, the client's getResultMessage() will return "SERVER IS MARKED DEAD." Further gets() for keys hashing to this server will continue returning this message, as will sets() hashing to the dead server.
Version 2.0.01b supports a constant Memcached::OPT_AUTO_EJECT_HOSTS which allows automatic rebalancing of the cluster/automatic failover handling.