diff --git a/railties/lib/generators/base.rb b/railties/lib/generators/base.rb
index e7d0ac80eb..d4bce83161 100644
--- a/railties/lib/generators/base.rb
+++ b/railties/lib/generators/base.rb
@@ -29,6 +29,39 @@ def self.namespace(name=nil) #:nodoc:
 
       protected
 
+        # Check whether the given class names are already taken by Ruby or Rails.
+        # In the future, expand to check other namespaces such as the rest of
+        # the user's app.
+        #
+        def class_collisions(*class_names)
+          return unless behavior == :invoke
+
+          class_names.flatten.each do |class_name|
+            class_name = class_name.to_s
+            next if class_name.strip.empty?
+
+            # Split the class from its module nesting
+            nesting = class_name.split('::')
+            last_name = nesting.pop
+
+            # Hack to limit const_defined? to non-inherited on 1.9
+            extra = []
+            extra << false unless Object.method(:const_defined?).arity == 1
+
+            # Extract the last Module in the nesting
+            last = nesting.inject(Object) do |last, nest|
+              break unless last.const_defined?(nest, *extra)
+              last.const_get(nest)
+            end
+
+            if last && last.const_defined?(last_name.camelize, *extra)
+              raise Error, "The name '#{class_name}' is either already used in your application " <<
+                           "or reserved by Ruby on Rails. Please choose an alternative and run "  <<
+                           "this generator again."
+            end
+          end
+        end
+
         # Use Rails default banner.
         #
         def self.banner
diff --git a/railties/lib/generators/rails/metal/metal_generator.rb b/railties/lib/generators/rails/metal/metal_generator.rb
index ba062b30be..f8833ecec3 100644
--- a/railties/lib/generators/rails/metal/metal_generator.rb
+++ b/railties/lib/generators/rails/metal/metal_generator.rb
@@ -1,7 +1,13 @@
-module Rails::Generators
-  class MetalGenerator < NamedBase
-    def create_file
-      template "metal.rb", "app/metal/#{file_name}.rb"
+module Rails
+  module Generators
+    class MetalGenerator < NamedBase
+      def check_class_collision
+        class_collisions class_name
+      end
+
+      def create_file
+        template "metal.rb", "app/metal/#{file_name}.rb"
+      end
     end
   end
 end
diff --git a/railties/lib/generators/rails/observer/observer_generator.rb b/railties/lib/generators/rails/observer/observer_generator.rb
index 9a5253a0ad..e5e1be39dc 100644
--- a/railties/lib/generators/rails/observer/observer_generator.rb
+++ b/railties/lib/generators/rails/observer/observer_generator.rb
@@ -1,8 +1,9 @@
 module Rails
   module Generators
     class ObserverGenerator < NamedBase
-      # TODO Check class collisions
-      # class_collisions "#{class_name}Observer", "#{class_name}ObserverTest"
+      def check_class_collision
+        class_collisions "#{class_name}Observer"
+      end
 
       def create_observer_file
         template 'observer.rb', File.join('app/models', class_path, "#{file_name}_observer.rb")
diff --git a/railties/lib/generators/rails/plugin/plugin_generator.rb b/railties/lib/generators/rails/plugin/plugin_generator.rb
index eb36cea0f1..4dbb3bfc0d 100644
--- a/railties/lib/generators/rails/plugin/plugin_generator.rb
+++ b/railties/lib/generators/rails/plugin/plugin_generator.rb
@@ -7,8 +7,9 @@ class PluginGenerator < NamedBase
       class_option :with_generator, :type => :boolean, :aliases => "-g", :default => false,
                                     :desc => "When supplied creates generator base files."
 
-      # TODO Check class collision
-      # class_collision class_name
+      def check_class_collision
+        class_collisions class_name
+      end
 
       def create_root
         self.root = File.expand_path("vendor/plugins/#{file_name}", root)
diff --git a/railties/test/generators/generators_test_helper.rb b/railties/test/generators/generators_test_helper.rb
index 9462fc2214..1fcfdaebb9 100644
--- a/railties/test/generators/generators_test_helper.rb
+++ b/railties/test/generators/generators_test_helper.rb
@@ -12,12 +12,8 @@ def destination_root
   end
 
   def setup
+    rm_rf(destination_root)
     mkdir_p(destination_root)
-    rm_rf(destination_root)
-  end
-
-  def teardown
-    rm_rf(destination_root)
   end
 
   def test_truth
diff --git a/railties/test/generators/metal_generator_test.rb b/railties/test/generators/metal_generator_test.rb
index dfa948d14e..4e73883feb 100644
--- a/railties/test/generators/metal_generator_test.rb
+++ b/railties/test/generators/metal_generator_test.rb
@@ -9,10 +9,15 @@ def test_metal_skeleton_is_created
     assert_file "app/metal/foo.rb", /class Foo/
   end
 
+  def test_check_class_collision
+    content = capture(:stderr){ run_generator ["object"] }
+    assert_match /The name 'Object' is either already used in your application or reserved/, content
+  end
+
   protected
 
-    def run_generator(args=[])
-      silence(:stdout) { Rails::Generators::MetalGenerator.start ["foo"].concat(args), :root => destination_root }
+    def run_generator(args=["foo"])
+      silence(:stdout) { Rails::Generators::MetalGenerator.start args, :root => destination_root }
     end
 
 end
diff --git a/railties/test/generators/plugin_generator_test.rb b/railties/test/generators/plugin_generator_test.rb
index b4aa39ae5f..3f2b27c518 100644
--- a/railties/test/generators/plugin_generator_test.rb
+++ b/railties/test/generators/plugin_generator_test.rb
@@ -15,23 +15,28 @@ def test_plugin_skeleton_is_created
     ).each{ |path| assert_file path }
   end
 
+  def test_check_class_collision
+    content = capture(:stderr){ run_generator ["object"] }
+    assert_match /The name 'Object' is either already used in your application or reserved/, content
+  end
+
   def test_invokes_default_test_framework
     run_generator
     assert_file "vendor/plugins/plugin_fu/test/plugin_fu_test.rb"
   end
 
   def test_logs_if_the_test_framework_cannot_be_found
-    content = run_generator ["--test-framework=unknown"]
+    content = run_generator ["plugin_fu", "--test-framework=unknown"]
     assert_match /Could not find and invoke 'unknown:generators:plugin'/, content
   end
 
   def test_creates_tasks_if_required
-    run_generator ["--with-tasks"]
+    run_generator ["plugin_fu", "--with-tasks"]
     assert_file "vendor/plugins/plugin_fu/tasks/plugin_fu_tasks.rake"
   end
 
   def test_creates_generator_if_required
-    run_generator ["--with-generator"]
+    run_generator ["plugin_fu", "--with-generator"]
     assert_file "vendor/plugins/plugin_fu/generators/plugin_fu/templates"
 
     flag = /class PluginFuGenerator < Rails::Generators::NamedBase/
@@ -40,8 +45,8 @@ def test_creates_generator_if_required
 
   protected
 
-    def run_generator(args=[])
-      silence(:stdout) { Rails::Generators::PluginGenerator.start ["plugin_fu"].concat(args), :root => destination_root }
+    def run_generator(args=["plugin_fu"])
+      silence(:stdout) { Rails::Generators::PluginGenerator.start args, :root => destination_root }
     end
 
 end