On my Debian Squeeze system I was getting WRITE FAILURE errors. After debugging and finally tcpdump it seems that the problem was me adding the server 'localhost', which resolved to '::1' (ipv6) while the default memcached server on debian only listens to '127.0.0.1' (ipv4). DNS automatically prefers ipv6 over ipv4.
I added the server '127.0.0.1' instead and everything worked. You could also disable ipv6 or have memcached listen on ::1