I have no idea what you've read, but custom forms are supported in Outlook 2003 and even 2007. 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.