What you are looking for is Windows Integrated Authentication. Someone in an earlier post mentioned a Java library that can do this, and I finally found the reference: http://www.ioplex.com/jespa.html
I have not yet used this library (it's on my to-do list) so I cannot provide any further guidance as to how to integrate it into Portal.
I am working on a same project, configuring Jboss portal to authenticate from AD. How did you configure the user profile to read the user id from AD instead of DB? In my case, I was not able to log in as the "enabled" flag was not set. Thank you in advance.