Description of problem:
Set your system SELinux to disabled (not permissive) and run:
# mkdir /tmp/foo
Now it is:
# ls -ldZ /tmp/foo
drwxr-xr-x root root ?                                /tmp/foo

# python
Python 2.6.5 (r265:79063, Jan 21 2011, 12:09:10) 
[GCC 4.4.4 20100726 (Red Hat 4.4.4-13)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> from selinux import lgetfilecon, is_selinux_enabled
>>> lgetfilecon('/tmp/foo')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
OSError: [Errno 61] No data available


While on RHEL5.7 I get as result:
[-1, None]
and on Fedora 14:
[10, 'unlabeled']

I would expect to have on RHEL6 either None or 'unlabeled', but I do not expect exception.

It block BZ 688461, but we will probably workaround it using try/except as we could not wait for correct fix ... unless it will make it to RHEL6.1