It's been stable for a while actually, but some people were hesitant to use it in production while it still had the beta designation. Well, no more, go forth and use it.
Future plans include the addition of memcached_dump and auto-ejection support. The memcached extension is a PHP library that allows your application to interface directly with a memcached server. Memcached is a high-performance, distributed memory object caching system, generic in nature, but intended for use in speeding up dynamic web applications by alleviating database load.