In trying to get this pluging working on our test site I came across a possible type in the function mentioned above.
The function shibboleth_session_active checks the array
for a couple of keys and uses that to determine if a session is active.
In the 1.4 code the keys which are checked are
$session_headers = array('Shib-Session-ID', 'HTTP_SHIB_IDENTITY_PROVIDER');
What I got to work here was a check using the following:
$session_headers = array('Shib_Session_ID', 'HTTP_SHIB_IDENTITY_PROVIDER');
Thought you might want to know.