Unfortunately this takes a bit of work, the one in the etch repositiories is too old so you're going to have to build you're own sources and install manually.

Add Repository / Increase Cache Limit

So first off add the testing repository to your list of sources

cd /etc/apt
sudo nano sources.list

Then add the testing repository (here for the UK) and save the file

deb-src http://ftp.uk.debian.org/debian/ testing main

Now before you update your sources list increase the size of the package cache by,

nano /etc/apt/apt.conf

And add,

APT::Cache-Limit "16777216";

...now when you update your packages cache you shouldn't get an out-of-space error, finally update

sudo apt-get update

Build Packages

First create & move to your sources dir

mkdir /sources
cd /sources

Next grab the tools needed to compile the sources

sudo aptitude install devscripts

Next we'll check if we need anything extra for libevent

sudo apt-get build-dep libevent

And grab & build the libevent source

sudo apt-get source libevent
cd libevent-1.3e
sudo debuild -us -uc
cd ..

Then install libevent and it's development package

sudo dpkg -i libevent1_1.3e-3_amd64.deb
sudo dpkg -i libevent-dev_1.3e-3_amd64.deb

Next we'll test if there are any build dependencies for memcached

sudo apt-get build-dep memcached

Then grab & build the memcached source

sudo apt-get source memcached
cd memcached-1.2.2
sudo debuild -us -uc
cd ..

Finally install memcached

sudo dpkg -i memcached_1.2.2-1_amd64.deb

You should now have a working install of Memcached 1.2.2 on Debian Etch.