First - strange, I am subscribed to this thread but haven’t received any emails and thank you for the responses
The question has nothing to do with the actual site running (I can access the site equally as well from domain.com and www.domain.com), it is specific to Twitter or rather the implementation of Twitter.
The question is this - what, if anything, do I need to do when I setup the Twitter application so that domain.com and www.domain.com use the same app (same consumer key and same consumer secret).
The problem occurs during the login to Twitter in that the final callback occurs to www.domain.com/authcallback.php even though the request started from domain.com. However, due to the age of the library used (I inherited this part of the project) I am not seeking help in how the library may be broken but rather how to configure a Twitter app for domain.com and www.domain.com.