Very interesting question that has lots of useful links on persistent object / value stores that are similar to memcache. Answer seems to be couchbase.
This says it all, couchbase includes memcached as part of it, and can operate in "pure memcache" mode or can add persistence and other features behind memcached. Sounds like it'll do what we're after.
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.