Memcached and Rails 3

I am porting a v2 app to v3 and hitting an issue with configuring
memcached as the cache store via:

    config.cache_store = :mem_cache_store, Memcached::Rails.new

In the new version this generates:

    /Users/neil/code/gli/config/environments/development.rb:20:in
`block in <top (required)>': uninitialized constant
Gli::Application::Memcached (NameError)

I have memcached in my Gemfile

Does anyone know how to correctly configure this?