Memcache provides no features related to key sharding, hashing, replication, HA, redundancy, or failover.
It's the client libraries that leverage some type of hashing algorithm to distribute keys across a cluster of memcached servers, but that functionality is completely independent from memcached itself.
There's lots of client libraries depending on your target language, just pick one that provides the hashing features you need and you're good to go:
http://code.google.com/p/memcached/wiki/Clients