I\'m attempting to implement a simple Single Sign On scenario where some of the participating servers will be windows (IIS) boxes. It looks like SPNEGO is a reasonable path
This is exactly what Apple does with its Calendar Server. They have a python gssapi library for the kerberos part of the process, in order to implement SPNEGO.
Look in CalendarServer/twistedcaldav/authkerb.py for the server auth portion. The kerberos module (which is a c module), doesn't have any useful docstrings, but PyKerberos/pysrc/kerberos.py has all the function definitions.
Here's the urls for the svn trunks:
http://svn.calendarserver.org/repository/calendarserver/CalendarServer/trunk
http://svn.calendarserver.org/repository/calendarserver/PyKerberos/trunk