Index: fields.py
===================================================================
--- fields.py	(revision 6059)
+++ fields.py	(working copy)
@@ -185,6 +185,8 @@
         super(DecimalField, self).clean(value)
         if not self.required and value in EMPTY_VALUES:
             return None
+        if isinstance(value, Decimal):
+            return value
         value = value.strip()
         try:
             value = Decimal(value)