WPMU / Multisite rediect question

It looks like by default any reference to mydomain.com automatically redirects to http://www.mydomain.com which is fine EXCEPT when a user of my multisite installation creates a link in their blog like this http://mydomains.com/username/page

When this occurs the site automatically redirects to the root domain of http://mydomain.com

If the user uses a www like this http://www.mydomains.com/username/page it works fine…

what gives?