1) What is the best resource/tutorial for learning how to set up memcached with Rails?
http://dev.robotcoop.com/Libraries/memcache-client/index.html
There's also some plugins that will let you use memcache for sessions/cache/models, etc...
2) Is the memcached server accessible to many client _machines_ so that my cache can be available to all of the servers in a Mongrel cluster?
Yes.