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 and suggests best practices.

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Sue Mosher, Outlook MVP
Author of Configuring Microsoft Outlook 2003
http://www.turtleflock.com/olconfig/index.htm
and Microsoft Outlook Programming - Jumpstart for
Administrators, Power Users, and Developers
http://www.outlookcode.com/jumpstart.aspx
> When employees at our company are using cached mode, custom message forms
> (*.oft files) do not show up. Originally when they clicked on the form it
[quoted text clipped - 3 lines]
> using OUTLOOK 2000 do not have this problem. Any suggestions are greatly
> appreciated.
LeoBloom - 08 Mar 2006 15:24 GMT
Thanks for your help. Not only did this help people using cached exchange
mode but PDA device users as well.
> 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.
>
[quoted text clipped - 7 lines]
> > using OUTLOOK 2000 do not have this problem. Any suggestions are greatly
> > appreciated.