I've got two FreeBSD buildslaves running on ZFS (8.2 and 9.1) experiencing this:

======================================================================
FAIL: test_futimes_ns (test.test_os.StatAttributeTests)
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Traceback (most recent call last):
  File "/home/buildslave/cpython/3.x.snakebite-freebsd91-amd64/build/Lib/test/test_os.py", line 369, in test_futimes_ns
    self._test_utime_ns(futimes_ns, test_dir=False)
  File "/home/buildslave/cpython/3.x.snakebite-freebsd91-amd64/build/Lib/test/test_os.py", line 339, in _test_utime_ns
    self._test_utime(self.fname, getattr_ns, set_times_ns, ten_s)
  File "/home/buildslave/cpython/3.x.snakebite-freebsd91-amd64/build/Lib/test/test_os.py", line 318, in _test_utime
    self.assertEqual(attr(st0, "st_mtime"), attr(st1, "st_mtime"))
AssertionError: 1345497270884503433 != 1345497270884503000

======================================================================
FAIL: test_lutimes_ns (test.test_os.StatAttributeTests)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/buildslave/cpython/3.x.snakebite-freebsd91-amd64/build/Lib/test/test_os.py", line 362, in test_lutimes_ns
    self._test_utime_ns(lutimes_ns)
  File "/home/buildslave/cpython/3.x.snakebite-freebsd91-amd64/build/Lib/test/test_os.py", line 339, in _test_utime_ns
    self._test_utime(self.fname, getattr_ns, set_times_ns, ten_s)
  File "/home/buildslave/cpython/3.x.snakebite-freebsd91-amd64/build/Lib/test/test_os.py", line 318, in _test_utime
    self.assertEqual(attr(st0, "st_mtime"), attr(st1, "st_mtime"))
AssertionError: 1345497270888847634 != 1345497270888847000

======================================================================
FAIL: test_utime (test.test_os.StatAttributeTests)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/buildslave/cpython/3.x.snakebite-freebsd91-amd64/build/Lib/test/test_os.py", line 331, in test_utime
    self._test_utime(self.fname, getattr, utime, 10)
  File "/home/buildslave/cpython/3.x.snakebite-freebsd91-amd64/build/Lib/test/test_os.py", line 318, in _test_utime
    self.assertEqual(attr(st0, "st_mtime"), attr(st1, "st_mtime"))
AssertionError: 1345497270.8923829 != 1345497270.892382

======================================================================
FAIL: test_utime_ns (test.test_os.StatAttributeTests)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/buildslave/cpython/3.x.snakebite-freebsd91-amd64/build/Lib/test/test_os.py", line 346, in test_utime_ns
    self._test_utime_ns(utime_ns)
  File "/home/buildslave/cpython/3.x.snakebite-freebsd91-amd64/build/Lib/test/test_os.py", line 339, in _test_utime_ns
    self._test_utime(self.fname, getattr_ns, set_times_ns, ten_s)
  File "/home/buildslave/cpython/3.x.snakebite-freebsd91-amd64/build/Lib/test/test_os.py", line 318, in _test_utime
    self.assertEqual(attr(st0, "st_mtime"), attr(st1, "st_mtime"))
AssertionError: 1345497270894558518 != 1345497270894558000


I briefly stepped through the test on one of the affected slaves.  There's a rounding issue somewhere... I personally think this method is involved:

    def _test_utime_ns(self, set_times_ns, test_dir=True):
        def getattr_ns(o, attr):
            return getattr(o, attr + "_ns")
        ten_s = 10 * 1000 * 1000 * 1000
        self._test_utime(self.fname, getattr_ns, set_times_ns, ten_s)
        if test_dir:
            self._test_utime(support.TESTFN, getattr_ns, set_times_ns, ten_s)

I didn't have enough time to keep debugging, so I'm raising this as a placeholder until I can pick it back up again.