> Just what App Engine already offers

No, it doesn't.

"The Memcache Python API High performance scalable web applications often use a distributed in-memory data cache in front of or in place of robust persistent storage for some tasks"

MySQL memcached is not a cache, it's a persistent storage with memcached API. And it's fast.

Isn't this MySQL memcached thing just a memcached in front of MySQL where the 'key' is the expanded SELECT statement?

What you've described it's MySQL query cache.

MySQL memcached is a memcached deamon that uses InnoDB storage instead of in-memory storage. The caching is done by InnoDB itself.

(and actually it's possible to enable the optional in-memory storage too)

http://dev.mysql.com/doc/refman/5.6/en/innodb-memcached.html