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