Tosh Meston mentioned this on his blog a while ago, but as OWA customization subjects seem to be very popular, here it is:
There are two relatively new TechNet articles out there that talk specifically about customizations of Exchange 2003 OWA in lots of detail:
Creating and Deploying Outlook Web Access Themes
Customizing the Outlook Web Access Logon Page
Have good OWA-moding!
- Nino Bilic