Index: lib/http/conditions.rb
===================================================================
--- lib/http/conditions.rb	(revisione 28)
+++ lib/http/conditions.rb	(copia locale)
@@ -15,6 +15,17 @@
     end
 
     alias_method '<<', :push
+    
+=begin rdoc
+Accessor to an array of additional elements to consider for Etag header's creation.
+For example, using ActionController::MimeResponds::InstanceMethods#respond_to for serving different
+content depending on request 'Accept' header, it's a good idea to include the used template 
+in Etag value:
+conditions.consider_for_etag << template
+=end    
+    def consider_for_etag
+      @consider_for_etag ||= []
+    end
 
     def test!
       return unless @models.size > 0
@@ -31,7 +42,9 @@
         end
       end
 
-      response.etag          = Digest::MD5.hexdigest(lock_versions.join('|'))
+      #response.etag          = Digest::MD5.hexdigest(lock_versions.join('|'))
+      etag_values = [].concat(consider_for_etag).concat(lock_versions)
+      response.etag          = Digest::MD5.hexdigest(etag_values.join('|'))
       response.last_modified = updated_at_dates.compact.sort.last || Time.now
 
       raise HTTP::PreconditionFailedError                                                          unless match? || unmodified_since?