acts_as_cached is a plugin by Chris Wanstrath, Tim Myrtle, and PJ Hyett that simply allows you to cache any Ruby object in memory (using memcached). Check out the documentation for the full instructions, but if you've got a working memcached server ready and waiting, it's as easy as installing a gem (memcached-client), a plugin (acts_as_cached), and adding 'acts_as_cached' to your model.

Chris gives a full run down of acts_as_cached in this blog post, and also links to a great PDF presentation about the plugin.