Fixing "uninitialized constant ActiveSupport::Dependencies::Mutex (NameError)"

RubyGems 1.6.0 has undergone some changes which may cause Rails 2.x applications to break with an error like this one (e.g. when running script/server):
/usr/lib/ruby/gems/1.8/gems/activesupport-2.3.8/lib/active_support/dependencies.rb:55: uninitialized constant ActiveSupport::Dependencies::Mutex (NameError)

Fix it by adding the following not only to your environment.rb but also into your script/server file, in both cases before boot.rb is required.
require 'thread'

If you still get the error above you can also add require 'thread' in your boot.rb after the RAILS_ROOT constant was defined.
RAILS_ROOT = "#{File.dirname(FILE)}/.." unless defined?(RAILS_ROOT)
require 'thread'