Following program:
------------------------------ notimpl.py
class CClassic:
  def __add__(self, other):
    return NotImplemented
  def __mul__(self, other):
    return NotImplemented
class CNew(object):
  def __add__(self, other):
    return NotImplemented
  def __mul__(self, other):
    return NotImplemented

a=CClassic()
try:
  print a+2
except Exception, e:
  print e
try:
  print a*2
except Exception, e:
  print e
a=CNew()
try:
  print a+2
except Exception, e:
  print e
try:
  print a*2
except Exception, e:
  print e
--------------------------------
Output following (correct) under Python 2.2:

unsupported operand types for +: 'instance' and 'int'
unsupported operand type(s) for *: 'instance' and 'int'
unsupported operand types for +: 'CNew' and 'int'
unsupported operand type(s) for *: 'CNew' and 'int'

And following (wrong) under Python 2.3[.2]:

unsupported operand type(s) for +: 'instance' and 'int'
unsupported operand type(s) for *: 'instance' and 'int'
unsupported operand type(s) for +: 'CNew' and 'int'
NotImplemented