Apache + Mongrel Cluster Caching (cross server)

Action caching can be configured to use a shared fragment store - DRb
or Memcache fragment stores are supplied. Expiry from any server will
expiry the action for all servers, and, with Memcache, the store can
be distributed over a set of servers for redundancy.

Page caching will need some form of sharing of the cache directory
across machines - NFS would be the obvious choice, but I don't know
how reliable this is, or how Rails or Apache behave when there are
network problems.