The following patch solves the problem (with the latest from trunk). I
only tested on Linux. I couldn't reproduce the problem with latest py3k
(again, on Linux).

===================================================================
--- Lib/os.py   (revision 58221)
+++ Lib/os.py   (working copy)
@@ -446,6 +446,9 @@
                 def __delitem__(self, key):
                     unsetenv(key)
                     del self.data[key.upper()]
+                def pop(self, key):
+                    unsetenv(key)
+                    return UserDict.IterableUserDict.pop(self, key)
                 def clear(self):
                     for key in self.data.keys():
                         unsetenv(key)
@@ -513,6 +516,9 @@
                         del self.data[key]
             def copy(self):
                 return dict(self)
+            def pop(self, key):
+                unsetenv(key)
+                return UserDict.IterableUserDict.pop(self, key)
 
 
     environ = _Environ(environ)