I find similar speedups:

isinstance(42, int)       -2%
isinstance(42, type)      45%
isinstance(object, type)  -1%
isinstance(object, int)   42%
isinstance(float, int)    44%

(both negative ones are actually lower than original, but so low that it
could be considered timing glitches)

However, also I have an error in the test_exceptions.py suite:
facundo@pomcat:~/devel/reps/python/trunk$ ./python
Lib/test/test_exceptions.py
testAttributes (__main__.ExceptionTests) ... ok
testInfiniteRecursion (__main__.ExceptionTests) ... FAIL
testKeywordArgs (__main__.ExceptionTests) ... ok
testRaising (__main__.ExceptionTests) ... ok
testReload (__main__.ExceptionTests) ... ok
testSettingException (__main__.ExceptionTests) ... ok
testSlicing (__main__.ExceptionTests) ... ok
testSyntaxErrorMessage (__main__.ExceptionTests) ... ok
testUnicodeStrUsage (__main__.ExceptionTests) ... ok
test_WindowsError (__main__.ExceptionTests) ... ok

======================================================================
FAIL: testInfiniteRecursion (__main__.ExceptionTests)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "Lib/test/test_exceptions.py", line 336, in testInfiniteRecursion
    self.assertRaises(RuntimeError, g)
AssertionError: RuntimeError not raised

----------------------------------------------------------------------
Ran 10 tests in 0.031s

FAILED (failures=1)
Traceback (most recent call last):
  File "Lib/test/test_exceptions.py", line 351, in <module>
    test_main()
  File "Lib/test/test_exceptions.py", line 348, in test_main
    run_unittest(ExceptionTests)
  File "/home/facundo/devel/reps/python/trunk/Lib/test/test_support.py",
line 577, in run_unittest
    _run_suite(suite)
  File "/home/facundo/devel/reps/python/trunk/Lib/test/test_support.py",
line 560, in _run_suite
    raise TestFailed(err)
test.test_support.TestFailed: Traceback (most recent call last):
  File "Lib/test/test_exceptions.py", line 336, in testInfiniteRecursion
    self.assertRaises(RuntimeError, g)
AssertionError: RuntimeError not raised