Index: actionpack/test/controller/url_rewriter_test.rb
===================================================================
--- actionpack/test/controller/url_rewriter_test.rb	(revision 3524)
+++ actionpack/test/controller/url_rewriter_test.rb	(working copy)
@@ -19,7 +19,9 @@
   def test_expand_array_build_query_string
     assert_query_equal '?x[]=1&x[]=2', @rewriter.send(:build_query_string, :x => [1, 2])
   end
-
+  def test_multidimensional_hashes_build_query_string
+    assert_query_equal '?this%5Bis%5D%5Ba%5D=test&this%5Bis_also%5D%5Ba%5D=hash', @rewriter.send(:build_query_string, :this => { 'is' => {'a' => 'test'}, 'is_also' => {'a' => 'hash'} })
+  end
   def test_escape_spaces_build_query_string_selected_keys
     assert_query_equal '?x=hello+world', @rewriter.send(:build_query_string, {:x => 'hello world', :y => 'goodbye world'}, [:x])
   end
Index: actionpack/lib/action_controller/url_rewriter.rb
===================================================================
--- actionpack/lib/action_controller/url_rewriter.rb	(revision 3524)
+++ actionpack/lib/action_controller/url_rewriter.rb	(working copy)
@@ -55,6 +55,8 @@
         query_string = ""
 
         only_keys ||= hash.keys
+
+        flatten_query_hash!(hash, only_keys)
         
         only_keys.each do |key|
           value = hash[key] 
@@ -70,5 +72,20 @@
         query_string << ("?" + elements.join("&")) unless elements.empty?
         query_string
       end
+      
+      # Flattens the query hash so multidimensional hashes in the params hash work properly.
+      def flatten_query_hash!(hash, only_keys)
+        hash.each do |k,v|
+          next unless v.kind_of? Hash
+          v.each do |n,o|
+            hash["#{k}[#{n}]"] = o
+            only_keys << "#{k}[#{n}]"
+          end
+          hash.delete(k)
+          only_keys.delete(k)
+          flatten_query_hash!(hash, only_keys)
+        end
+      end
+      
   end
 end