diff --git a/activeresource/CHANGELOG b/activeresource/CHANGELOG
index ef3f3f6a17..c81f7daacb 100644
--- a/activeresource/CHANGELOG
+++ b/activeresource/CHANGELOG
@@ -1,5 +1,14 @@
 *SVN*
 
+* Optimistic locking: raise ActiveResource::ResourceConflict on 409 Conflict response. [Jeremy Kemper]
+
+    # Example controller action
+    def update
+      @person.save!
+    rescue ActiveRecord::StaleObjectError
+      render :xml => @person.reload.to_xml, :status => '409 Conflict'
+    end
+
 * Basic validation support [Rick Olson]
 
   Parses the xml response of ActiveRecord::Errors#to_xml with a similar interface to ActiveRecord::Errors.  
diff --git a/activeresource/lib/active_resource/connection.rb b/activeresource/lib/active_resource/connection.rb
index 132e861292..21573a34cb 100644
--- a/activeresource/lib/active_resource/connection.rb
+++ b/activeresource/lib/active_resource/connection.rb
@@ -17,14 +17,12 @@ module ActiveResource
     end
   end
 
-  class ClientError < ConnectionError
-  end
+  class ClientError < ConnectionError;  end  # 4xx Client Error
+  class ResourceNotFound < ClientError; end  # 404 Not Found
+  class ResourceConflict < ClientError; end  # 409 Conflict
 
-  class ServerError < ConnectionError
-  end
+  class ServerError < ConnectionError;  end  # 5xx Server Error
 
-  class ResourceNotFound < ClientError
-  end
 
   class Connection
     attr_accessor :site
@@ -73,6 +71,8 @@ module ActiveResource
             raise(ResourceNotFound.new(response))
           when 400
             raise(ResourceInvalid.new(response))
+          when 409
+            raise(ResourceConflict.new(response))
           when 401...500
             raise(ClientError.new(response))
           when 500...600
diff --git a/activeresource/test/base_test.rb b/activeresource/test/base_test.rb
index 97f2ea3d01..d8c654350b 100644
--- a/activeresource/test/base_test.rb
+++ b/activeresource/test/base_test.rb
@@ -133,6 +133,14 @@ class BaseTest < Test::Unit::TestCase
     addy.save
   end
 
+  def test_update_conflict
+    ActiveResource::HttpMock.respond_to do |mock|
+      mock.get "/people/2.xml", @david
+      mock.put "/people/2", nil, 409
+    end
+    assert_raises(ActiveResource::ResourceConflict) { Person.find(2).save }
+  end
+
   def test_destroy
     assert Person.find(1).destroy
     ActiveResource::HttpMock.respond_to do |mock|
diff --git a/activeresource/test/connection_test.rb b/activeresource/test/connection_test.rb
index d72c1c612d..0744e327e6 100644
--- a/activeresource/test/connection_test.rb
+++ b/activeresource/test/connection_test.rb
@@ -20,6 +20,9 @@ class ConnectionTest < Test::Unit::TestCase
     # 400 is a validation error
     assert_response_raises ActiveResource::ResourceInvalid, 400
 
+    # 409 is an optimistic locking error
+    assert_response_raises ActiveResource::ResourceConflict, 409
+
     # 4xx are client errors.
     [401, 499].each do |code|
       assert_response_raises ActiveResource::ClientError, code