When using more than 1 mc backend servers the delete operations seem to fail.
On update/save post and other operations like approving comments it does not seem to be deleting the key properly. On page refresh after the POST (say post update) the same data is returned as if the memcached key was not deleted.
When we were using 1 mc server in the pool it was fine, now with 3 servers it is no bueno.
Using v 2.0.2 of the plugin and a salt like:
define( 'WP_CACHE_KEY_SALT', md5(<some_unique_site_identifier>) );