Dependency Injection: Vitally Important or Totally Irrelevant? by Jim Weirich at OSCON 2005.
This is a great presentation about Dependency of Inversion (and why you might not need it). It nicely compares and contrasts Java and Ruby. It underscores why I really enjoy programming in Ruby: it is very fluid. For example, classes are objects and can change at runtime.
It also answers the question: Do you need something like Spring or HiveMind in Ruby?