I'm working on a Rails plugin called acts_as_fox. It overrides every method on an ActiveRecord model to return the string "chunky bacon!".
I got it working, thanks to the ruby-talk list, but ran into some kind of edge case, and have been too busy to get it wrapped up. There's nothing actually useful about this project, it's just a programming exercise.
Check out the thread on ruby-talk; the replies by Rick DeNatale and David Black are, as always, especially helpful.