For RoR, you can override to_param on the model in question, something like:

  class User < ActiveRecord::Base
    def to_param
      self.login
    end
    ...
  end

Assuming self.login is reliably unique. Although I've found this can still have some follow on effects that could need to be handled in a situation similar to the above. There are also some heavier weight, more feature filled options of course.