UPDATE: this is not mantined nor official,you should use the couchbase or other builds
After spending a day searching for a Memcached Windows 64-bit build ,I finally gave up and compile it by myself.
I manage to build it based on Memcached 1.4.2 with service code from:http://jehiah.cz/projects/memcached-win32/
The build platform is mingw64(google it) and regular mingw for configuring(as mingw64 get recognize as cygwin).
This is true 64-bit,running stats will show you the pointer_size is 64 and you can make the memcached process go over 4GB.