how to bypass authentication in ntlmhttpfilter?

Hi,

I have got automatic logon to a web application using NT credentials.  I allow 
users to logoff the webapp by setting a session attribute when a particular 
request header is sent, so that the NTLM filter is not executed (and thus the 
user can browse the site anonymously).

I would also like to implement the following:

If a user cannot automatically authenticate against the domain, do NOT show the 
network dialog.  (i.e. allow users to transparently fail NTLM and then browse 
the site anonymously).  

Has anyone done this?  Any hints appreciated!


thanks
jim