Nathan Neulinger made a patch for memcached, which adds it ability to work over local unix sockets. This might give memcached one more speed-boost in one-server cases. Patches address server-part and perl-based client part. It shouldn’t be difficult to make similiar patch for PHP
Original note can be found at gmane.comp.web.cache.memcached.
This patch didn’t have official response yet, as Brad Fitzpatrick is, supposedly, away on his honeymoon.