Hi,
I am the author of localmemcache. I wrote it so I could set view
counters and last-access dates in a webserver module in C and still
being able to access this information when I am in Ruby without having
to resort to storing these things in a real database (a huge speed
improvement). I was asked by people if I know other use cases of
using it with rails, but couldn't think of any--but I am curious about
hearing other ideas. 
http://localmemcache.rubyforge.org/
Best,
Sven