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.
Very useful article outlining the broad basics of optimising Magento. Includes links to more detailed information and lots of config file examples. Very useful.
Very detailed article explaining how Magento implements its caching mechanism. It seems to be some custom classes layered on top of Zend's caching framework. Useful read.
Excellent article describing in depth how to have PHP store session data in memcached and the differences between the memcache and memcached implementations in PHP. Very useful.
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.