Index: django/oldforms/__init__.py
===================================================================
--- django/oldforms/__init__.py	(revision 4431)
+++ django/oldforms/__init__.py	(working copy)
@@ -365,7 +365,7 @@
 
     def get_id(self):
         "Returns the HTML 'id' attribute for this form field."
-        return FORM_FIELD_ID_PREFIX + self.field_name
+        return FORM_FIELD_ID_PREFIX + self.field_name.replace('.', '_')
 
 ####################
 # GENERIC WIDGETS  #