I don't see any issue here, runs perfectly fine on Mac OS X (Snow Leopard)

Shashwat-Anands-MacBook-Pro:test l0nwlf$ pwd
/Users/l0nwlf/python-svn/Lib/test
Shashwat-Anands-MacBook-Pro:test l0nwlf$ python2.7 test_posix.py
testNoArgFunctions (__main__.PosixTester) ... ok
test_access (__main__.PosixTester) ... ok
test_chdir (__main__.PosixTester) ... ok
test_chflags (__main__.PosixTester) ... ok
test_chown (__main__.PosixTester) ... ok
test_confstr (__main__.PosixTester) ... ok
test_dup (__main__.PosixTester) ... ok
test_dup2 (__main__.PosixTester) ... ok
test_fchown (__main__.PosixTester) ... ok
test_fdopen (__main__.PosixTester) ... ok
test_fstat (__main__.PosixTester) ... ok
test_fstatvfs (__main__.PosixTester) ... ok
test_ftruncate (__main__.PosixTester) ... ok
test_getcwd_long_pathnames (__main__.PosixTester) ... ok
test_initgroups (__main__.PosixTester) ... ok
test_lchflags (__main__.PosixTester) ... ok
test_lchown (__main__.PosixTester) ... ok
test_lsdir (__main__.PosixTester) ... ok
test_osexlock (__main__.PosixTester) ... ok
test_osshlock (__main__.PosixTester) ... ok
test_pipe (__main__.PosixTester) ... ok
test_stat (__main__.PosixTester) ... ok
test_statvfs (__main__.PosixTester) ... ok
test_strerror (__main__.PosixTester) ... ok
test_tempnam (__main__.PosixTester) ... ok
test_tmpfile (__main__.PosixTester) ... ok
test_umask (__main__.PosixTester) ... ok
test_utime (__main__.PosixTester) ... ok

----------------------------------------------------------------------
Ran 28 tests in 0.025s

OK

Shashwat-Anands-MacBook-Pro:test l0nwlf$ python2.7 --versionPython 2.7a3+
Shashwat-Anands-MacBook-Pro:test l0nwlf$ python2.7 
Python 2.7a3+ (trunk:78165, Feb 12 2010, 22:36:03) 
[GCC 4.2.1 (Apple Inc. build 5646)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import posix
>>> posix.getgroups()
[20, 204, 100, 98, 81, 80, 79, 61, 12, 402, 401]
>>>