Beginning with Outlook 2003 SP2, users can work with most .oft form templates files only by opening the template from the Tools | Forms | Choose Form dialog, browsing for User Templates in File System. This means that if the .oft file is sent as a message attachment, the user must save it to the local hard drive first.

The article at http://support.microsoft.com/?id=907985 explains this change in more detail, suggests best practices, and provides registry keys that can return Outlook 2003 to the older, less secure behavior.