marshall.diff is my workaround (you probably want a better solution), rubygems.diff is how you disable the local cache completely. :)

marshall.diff

diff --git a/kernel/common/marshal.rb b/kernel/common/marshal.rb
index 688c24c..f9f9823 100644
--- a/kernel/common/marshal.rb
+++ b/kernel/common/marshal.rb
@@ -546,7 +546,7 @@ module Marshal
 
       call obj if @proc and call_proc
 
-      @stream.tainted? ? obj.taint : obj
+      (@stream.tainted? && !obj.frozen?) ? obj.taint : obj
     end
 
     def construct_class

rubygems.diff

diff --git a/lib/rubygems/source.rb b/lib/rubygems/source.rb
index f0e2a59..c0bdd3e 100644
--- a/lib/rubygems/source.rb
+++ b/lib/rubygems/source.rb
@@ -83,26 +83,26 @@ class Gem::Source
 
     cache_dir = cache_dir uri
 
-    local_spec = File.join cache_dir, spec_file_name
+    #local_spec = File.join cache_dir, spec_file_name
 
-    if File.exist? local_spec then
-      spec = Gem.read_binary local_spec
-      spec = Marshal.load(spec) rescue nil
-      return spec if spec
-    end
+    #if File.exist? local_spec then
+    #  spec = Gem.read_binary local_spec
+    #  spec = Marshal.load(spec) rescue nil
+    #  return spec if spec
+    #end
 
     uri.path << '.rz'
 
     spec = fetcher.fetch_path uri
     spec = Gem.inflate spec
 
-    if update_cache? then
-      FileUtils.mkdir_p cache_dir
+    #if update_cache? then
+    #  FileUtils.mkdir_p cache_dir
 
-      open local_spec, 'wb' do |io|
-        io.write spec
-      end
-    end
+    #  open local_spec, 'wb' do |io|
+    #    io.write spec
+    #  end
+    #end
 
     # TODO: Investigate setting Gem::Specification#loaded_from to a URI
     Marshal.load spec