I have RPC over HTTPS set up as well as OWA 2003. With OWA, i was able to edit the logon.asp page so that users would only need to enter "username" as opposed to "domain\username". It works great. Now ...
When using Outlook offsite in RPC over HTTPS mode our users always have to use "DOMAIN\Username" format to login. Outlook remembers the username once you've entered it once, but it can be a problem ...