Using version 2.0.2 of object-cache.php. WP version 3.2.1.
I have a situation where I have a couple sites (a development site and a production site) using the same memcached server, but we don't run our WP sites in multisite for other reasons. It looks like to me that on line 383 that the $blog_id is only enabling $table_prefix if is_multisite is returning as true. This causes problems where our development server and production servers are getting are getting cross-talk in the data. This is really apparent when trying to log into wp-admin on production site is getting redirected to the development login page and vice versa. (In the meantime I've shut down the development site.) I don't know how else the sites were affected since just seeing problem alone was bad enough.
What would be great is if the $blog_id is defined in wp-config.php then the table prefix will always be enabled. (I'm defining the variables as shown at this link: http://wpscale.com/tag/wordpress-memcached-problem/)