diff -r 5d08c2a6274d Lib/copy.py
--- a/Lib/copy.py	Mon Jun 27 09:18:46 2011 -0500
+++ b/Lib/copy.py	Mon Jun 27 08:49:22 2011 -0700
@@ -161,6 +161,8 @@
     copier = _deepcopy_dispatch.get(cls)
     if copier:
         y = copier(x, memo)
+        if y is x:
+            return y
     else:
         try:
             issc = issubclass(cls, type)
@@ -246,7 +248,6 @@
             break
     else:
         y = x
-    memo[d] = y
     return y
 d[tuple] = _deepcopy_tuple
 
diff -r 5d08c2a6274d Lib/test/test_copy.py
--- a/Lib/test/test_copy.py	Mon Jun 27 09:18:46 2011 -0500
+++ b/Lib/test/test_copy.py	Mon Jun 27 08:49:22 2011 -0700
@@ -311,9 +311,24 @@
 
     def test_deepcopy_keepalive(self):
         memo = {}
-        x = 42
+        x = []
         y = copy.deepcopy(x, memo)
-        self.assertTrue(memo[id(x)] is x)
+        self.assertTrue(memo[id(memo)][0] is x)
+
+    def test_deepcopy_dont_memo_immutable(self):
+        memo = {}
+        x = [1, 2, 3, 4]
+        y = copy.deepcopy(x, memo)
+        self.assertEqual(y, x)
+        # There's the entry for the new list, and the keep alive.
+        self.assertEqual(len(memo), 2)
+
+        memo = {}
+        x = [(1, 2)]
+        y = copy.deepcopy(x, memo)
+        self.assertEqual(y, x)
+        # Tuples with immutable contents are immutable for deepcopy.
+        self.assertEqual(len(memo), 2)
 
     def test_deepcopy_inst_vanilla(self):
         class C: