Over the last few months there has been a lot of noise and questions regarding ADFS support in SharePoint products and Technologies. First we were going to support it in Windows Server 2003 R2, then we pulled the support & now we will be supporting certian funcationality. Bascially it boils down to Office client integration. There is some funcationality that just wont work without work done in the client ... primarily that is the stuff that is not supported now.
Read the full KB for a better explaination:
Windows SharePoint Services and SharePoint Portal Server 2003 Support boundaries for Active Directory Federation Services