I had a customer who was having a problem with a ISA published Exchange 2003 OWA. Phe problem had two symptoms:
1. When they clicked on an URL in a mail in OWA they where redirected to a login page.
2. When the create a new message and attach a file they get redirected to a login page.
Turns out this was tho separate problems:
This is a problem with authentication settings in IIS
This is a problem with the delegation to the backend Server. This is a known bug in ISA 2007. There is a hotfix but you have to call PSS to get it.