I just setup new OWA servers using Exchange 2003 FE NLB
When a user hits the owa site (using https only) it prompts for username and password.
We have 3 domains. 1 is the forest, which users can login without having to put the domain (lastname first name initial). When a user needs to access a mailbox o a subdomain, they have to put domain (UPN). I have set the default domain to the forest which all accounts have it in theri account login.