I don't use OWA but IE can store passwords for web sites. Do you have it enabled. I am not sure how Outlook authenticates but I am guessing it's like any other web page?
ed: The OWA issues an ASP login page upon recieving a secure connection to the server. There is no way to by pass the ASP without modification of the server.
West of House
You are standing in an open field west of a white house, with a boarded front door.
There is a small mailbox here.
With Exchange 5.5, you were able to create an html page with a logon script so users can log into webmail. It is also doable with Exchange 2000 but I do not know the code. Instead of having users being prompted with a network logon box, I'd like them to be directed to a webpage where they can log in from there.